JPH04368464A - Dc power source - Google Patents

Abstract

PURPOSE:To limit a switching loss generated in a transient state of a PWM switching of a semiconductor switch, to simplify a semiconductor cooling system and to enhance an upper limit of an allowable switching frequency in a DC power source. CONSTITUTION:A DC power source 1 is connected to a primary winding of a transformer T through a semiconductor switch, and a secondary winding of the transformer is connected to a smoothing filter 2 of a parallel circuit of a reactor Ld and a capacitor Cd through rectifying diodes D5-D8. A DC power source in which a parallel circuit of a diode and a semiconductor switch and a resonance switch 3 formed of a resonance capacitor CR connected in series with the parallel circuit are provided in parallel at the DC output side of the diodes D5-D8, in the DC power source for energizing a load RL from the capacitor Cd, is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC power supply device that outputs DC power.

0002

2. Description of the Related Art A DC/DC converter is used to adjust an unstable DC power supply voltage to a stable DC voltage, to change the voltage level, or to extract an isolated DC voltage from a DC power supply. This DC/DC converter performs the power conversion function by turning on and off the built-in semiconductor switch, but in actual semiconductor switches, it takes a considerable amount of time for the state transition between on and off. This results in switching loss. The switching loss of this semiconductor switch is controlled by pulse width control (hereinafter P
This will be explained by taking as an example a DC/DC converter that performs WM (WM). FIG. 9 shows the circuit configuration. In the figure, E is a DC power supply, Q1 to Q4 are semiconductor switches, and D1 to D8
is a diode, T is a transformer, Ld is a reactor, C
d is a capacitor and RL is a load. First, the operation of the circuit will be explained with reference to FIG. FIG. 10 shows a time chart of voltage, current, and signals applied to the semiconductor switch. In the figure, when signals q1 to q4 are applied to semiconductor switches Q1 to Q4 constituting an inverter circuit, the pairs of semiconductor switches Q1, Q4, and Q2
, Q3 alternately turn on and off to convert the voltage E of the DC power supply 1 into an AC voltage. This is the voltage Vab between points ab. This is applied to the primary winding of the transformer T, and the AC voltage induced in the secondary winding of the transformer is rectified into DC voltage by a rectifier diode. This is reactor Ld and capacitor C
d to the smoothing filter constructed by d. Reactor Ld
Since the current Id flowing through the rectifier diode D is continuous, the current flowing through the rectifier diode D becomes the current I0 when the semiconductor switches Q1 to Q4 are in the off state. The voltage of the capacitor Cd is applied to the load RL. The voltage applied to the load RL is determined by a function of the DC power supply voltage E and the on-state and off-state times of the semiconductor switches Q1 to Q4. Generally, it is determined by PWM control, that is, by controlling the ratio of on-time within a certain period.

Next, the mechanism of occurrence of switching loss in the semiconductor switches Q1 to Q4 constituting the inverter circuit will be explained using FIG. 10. In the figure, it is assumed that the semiconductor switch pair Q1 and Q4 are respectively applied with signals q1 and q4 and are in the on state, and the current Id of the reactor Ld is equal to the current Iab flowing from the DC power supply 1 (this In this case, the transformation ratio of the transformer T is assumed to be 1). The voltage Vab between points ab is equal to the level of the voltage E of the DC power supply. Signals q1 and q4 are set to zero at time t1. Semiconductor switches Q1 and Q4 transition to the switched off state over time. The voltage across the semiconductor switches Q1 and Q4 gradually increases, and therefore the voltage Vab gradually decreases. During this time, the current Id of the reactor Ld continues to flow through the semiconductor switches Q1 and Q4. When time t2 is reached, the voltage applied to semiconductor switches Q1 and Q4 becomes equal to DC power supply voltage E, and therefore voltage Vab becomes zero. Further, the current Iab that had been flowing from the semiconductor switches Q1 and Q4 becomes zero, and instead, the current Id flowing through the smoothing reactor Ld begins to flow through the diode D as a circulating current I0. This period from t1 to t2 is the semiconductor switch Q1
, Q4 is a transition period from on to off. During this period, a current Iab flows through the semiconductor switches Q1 and Q4, and a voltage is also applied, resulting in a power loss determined by (voltage x current). At time t3 after the off time has elapsed, the signals q2 and q3 are switched to the respective semiconductor switches Q.
2 and Q3 to turn them on, the current Iab flowing through the semiconductor switches Q2 and Q3 gradually increases from zero. The circulating current I0 continues to flow through the diode D until this current reaches the level of the current Id flowing through the smoothing reactor Ld. During this time, the voltage applied to the smoothing filter is at zero level (actually the voltage applied to the diode D
(approximately 2 volts corresponding to the voltage drop) is applied to semiconductor switches Q2 and Q3, and the DC power supply voltage
is on. Therefore, voltage Vab is still at zero. At time t4, the currents in the semiconductor switches Q2 and Q3 reach the current Id flowing through the smoothing reactor. However, diode D has the undesirable characteristic that immediately after current flows in the forward direction, current also flows in the reverse direction.
Even after time t4, the semiconductor switches Q2 and Q3 cause current to flow in the opposite direction to the diode D at the same time as Id flows through the smoothing reactor Ld, so that the current continues to increase. Time t5 for diode D to recover reverse characteristics
At this point, the current flowing through the semiconductor switches Q2 and Q3 becomes equal to Id. In addition, semiconductor switches Q2, Q
3 becomes zero (actually, a voltage of about 2 volts corresponding to the voltage drop of the semiconductor switches Q2 and Q3 is applied). Therefore, the voltage Vab is also established. This period from time t3 to t5 is the semiconductor switch Q2
, Q3 are in the process of transitioning from the off state to the on state, and current flows through the semiconductor switches Q2 and Q3 while the voltage E is applied thereto, resulting in power loss. After time t5, the semiconductor switches Q2 and Q3 are in the on state, and the DC power supply voltage E is transformed by the transformer and applied to the smoothing filter. Signal q2 after a certain period of time
, q3 are set to zero, the semiconductor switches Q2 and Q3 are turned off after a process similar to the time t1 to t2 described above. Next, when the signals q1 and q4 are applied to the semiconductor switches Q1 and Q4, the above-mentioned time t3 to t
It moves to the initial state through a process similar to 5.

[0004] As explained above, the semiconductor switches Q1 to Q4 of the inverter circuit incur power loss when switching between on and off states. These are collectively referred to as switching losses. At time t2, the current Iab flowing through the semiconductor switches Q1 and Q4 suddenly decreases. Further, at time t5, when diode D recovers its reverse characteristic, current Iab suddenly changes. At this time, a spike voltage corresponding to the product of the current change rate (dIab/dt) and the inductance in the wiring of the semiconductor switch is generated, and this is applied as an overvoltage to the semiconductor switches Q1 and Q4 that have been switched off. . This spike voltage may destroy the semiconductor switches Q1 and Q4 or increase the probability of failure. A similar overvoltage occurs when semiconductor switches Q2 and Q3 are turned off.

[0005]

[Problem to be Solved by the Invention] In the prior art, since there is a switching loss each time the semiconductor switches Q1 to Q4 of the inverter circuit are turned on and off, when switching is performed at a high frequency, the semiconductor switches Q1 to Q4
The amount of heat generated increases. This requires a large semiconductor cooling system, making the DC power supply large and reducing efficiency. On the other hand, if the amount of heat generated by the semiconductor switches Q1 to Q4 is suppressed, the switching frequency cannot be increased, so the smoothing filter must be made larger, which increases the size of the device and sacrifices controllability to some extent. Moreover, since the spike voltage applied to the semiconductor switches Q1 to Q4 becomes excessive, the reliability of the DC power supply device is reduced.

The present invention was proposed in order to improve the above-mentioned drawbacks, and its purpose is to suppress the switching loss that occurs in the transient state of PWM switching in the semiconductor switches Q1 to Q4 constituting the inverter circuit.
The purpose is to simplify the semiconductor cooling system and raise the upper limit of the allowable switching frequency.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the present invention connects a DC power supply and a primary winding of a transformer via a semiconductor switch, and connects a secondary winding of the transformer, a reactor, and a capacitor. In a DC power supply device that connects a smoothing filter constituted by a series circuit of , through a rectifier diode, and supplies power from the capacitor to a load, a parallel circuit of a diode and a semiconductor switch, and a resonant capacitor connected in series thereto. The gist of the invention is a DC power supply device characterized in that a resonant switch is provided in parallel on the DC output side of the rectifier diode. Furthermore, the present invention connects the DC power supply and the primary winding of a transformer via a semiconductor switch, and connects the secondary winding of the transformer and a smoothing filter constituted by a series circuit of a reactor and a capacitor via a rectifier diode. In a DC power supply device that supplies power from the capacitor to a load, a series circuit of a diode and a resonant capacitor is provided in parallel with the DC output of the rectifier diode, and a connection point between the resonant capacitor and the diode, a smoothing filter reactor, and a capacitor are connected. The gist of the invention is a DC power supply device characterized in that it is connected to a connection point via a snubber diode.

[0008]

[Function] The present invention provides a DC power supply device in which a parallel circuit of a diode and a semiconductor switch, and a resonant switch consisting of a resonant capacitor connected in series with the parallel circuit are provided in parallel on the DC output side of the rectifier diode. It has the effect of suppressing the switching loss that occurs during the switching transient state of the semiconductor switch.

[0009]

[Example] Next, an example of the present invention will be described. Note that the embodiments are merely illustrative, and it goes without saying that various changes and improvements can be made without departing from the spirit of the present invention.

FIG. 1 shows a first embodiment of the DC power supply device of the present invention. In the figure, 1 is a DC power supply, 2 is a smoothing filter, 3 is a resonance switch, and 4 is a control device, Q1 to Q4.
are semiconductor switches that constitute an inverter circuit, and rectifier diodes D1 to D4 are connected in parallel to each semiconductor switch.
is connected. The primary winding of the transformer T is connected between the connection point a between the semiconductor switches Q1 and Q2 and the connection point b between the semiconductor switches Q3 and Q4, and the secondary winding is connected to the diode D5 via the resonant reactor LR. ~D
The output of this bridge is applied to a load RL via a smoothing filter 2. Note that the resonant switch 3 is inserted between the output side of the rectifying bridge and the smoothing filter 2. The control device 4 provides on/off signals for the semiconductor switches Q1 to Q4 and QR. Note that the semiconductor switch is a bipolar transistor (Bipolar Transistor).
r), MOS/FET, thyristor, gate turn-off thyristor (GTO), IGBT (Insulat
ed Gate Bipolar Transistor
r ), etc., but here we will explain using a bipolar transistor as a representative.

FIG. 2 shows operational waveforms for explaining the embodiment of FIG. Iab is the current flowing between points a and b, Vab is the voltage between points a and b, IR is the current flowing to the resonant switch, VR is the voltage across the capacitor CR, I0 is the voltage from the rectifier bridge to the smoothing filter 2, and the load Rl
Indicates the current flowing in q1 to q4 and qR indicate control signals.

Next, the operation will be explained. Assume that on-signals q1 and q4 are applied from the control device 4, and the semiconductor switches Q1 and Q4 of the inverter circuit are in the on-state. A current Iab (equal to Id) flows from the DC power supply 1 to the load RL via the smoothing filter 2. At time t01, the semiconductor switch QR of the resonant switch 3
When turned on by the signal qR from the control device 4, current starts to flow from the DC power supply 1 to the resonant capacitor CR. The current IR at this time is a series resonant current of the resonant reactor LR and the resonant capacitor CR, and the flowing direction is opposite to the illustrated arrow. Semiconductor switch Q1, Q4
And the current flowing through the resonant reactor LR is the current Id
and IR, which increases in a pulsed manner. The resonant current of the resonant capacitor CR charges it to a level higher than the DC power supply voltage E (when the transformation ratio of the transformer is 1). Charging ends at time t02, and then the charge stored in the resonant capacitor CR begins to be discharged through the resonant diode DR and the resonant capacitor CR, and a current IR flows in the direction of the arrow. Current Id due to the action of reactor Ld
Since the magnitude is equal to the sum of the currents Iab and IR and is almost constant, as the current IR increases, the current Iab decreases. When current IR becomes equal to Id at time t03, current Iab becomes zero. The discharge progresses further and the time t
When the charge on the resonant capacitor CR becomes zero at 05, the current flowing through the resonant diode DR becomes zero. Since the current flowing through the resonant reactor Ld is continuous, when the current in the resonant diode DR becomes zero, the current Id switches from the diode DR to the diodes D5, D6 or D7, D8 and becomes the current I0 flowing through the diode D5, D6 or D7, D8.
becomes. This maintains the continuity of the current Id in the reactor Ld.

[0013] Resonant diode DR before time t05
When the semiconductor switches Q1 and Q4 are turned off by the signal qR from the control device 4 at time t04 when the semiconductor switches Q1 and Q are energized, the voltage Vab becomes zero, and the semiconductor switches Q1 and Q
A voltage approximately equal to the DC power supply voltage E is applied to 4. Since the currents in the semiconductor switches Q1 and Q4 have already become zero at time t03, no switching loss occurs during the switch-off process. On the other hand, when the semiconductor switch QR is turned on at time t01, the resonant reactor L
The current IR will gradually increase due to the current suppression effect of R, so that the current IR is still at a small level in the switching-on transient state. Therefore, the switching loss obtained by multiplying the voltage and current of the semiconductor switch is small. The resonant diode DR is energized and the current IR
is flowing in the direction shown in the figure, the semiconductor switch QR
When the semiconductor switch QR is turned off, the current in the semiconductor switch QR has already become zero, so there is no switching loss caused by turning it off.

At time t06, signals q2,
q3 is the semiconductor switch Q2, Q3 of the inverter circuit
to turn this on. At this point, reactor L
The current Id flowing through d is the current I0 flowing through the diodes D5 to D8. Even if the semiconductor switches Q2 and Q3 are turned on due to the action of the resonant reactor LR, the current Iab
does not increase rapidly. During the switching transition period, Id can be considered to be constant because the reactor Ld has a large constant. Since the sum of currents I0 and Iab is constant and equal to Id, Iab increases as I0 decreases. Since the transition period during which the semiconductor switches Q2 and Q3 are turned on ends while the level of the current Iab is small, the switching loss of the semiconductor switches Q2 and Q3 is small. After time t07 when the current I0 reaches zero, the reactor L
All of the current Id flowing through the semiconductor switch Q2
, Q3 becomes a current Iab flowing through them.

Next, when the semiconductor switch QR is turned on, the semiconductor switch Q goes through the same process as t01 and t05.
2, Q3 turns off. When semiconductor switches Q1 and Q4 are turned on after a certain period of time has elapsed, the above-mentioned t06 and t07
The process returns to the initial state. This completes one cycle of on/off of the semiconductor switches Q1 to Q4. This on-off cycle is then repeated, and power is sent from the DC power supply 1 to the load RL at a controlled voltage. As explained above, the semiconductor switches Q1 to Q4 of the inverter circuit complete switching while the current value flowing during on-switching is small, and when off-switching, the switching is completed while the flowing current value is small.
Since switching is completed when the current flowing is zero, the switching loss is only a small value. Similarly, the switching loss of the semiconductor switch QR is also small.

FIG. 3 shows a second embodiment of the invention. Figure 1
Semiconductor switches Q1 to Q4 in the first embodiment of
Instead of a bridge inverter circuit using a semiconductor switch, a conventionally known single-stone forward type DC-DC converter consisting of only Q01 as a semiconductor switch is used. This converter can supply power by converting power from winding n1 to n2 of the transformer only while Q01 is on. During the period when the semiconductor switch Q01 is off, the excitation energy stored in the transformer T is recovered to the DC power supply 1 from the winding n3. The voltage induced in the winding of the transformer T at this time has a polarity opposite to that during the period when the semiconductor switch Q01 is on, and the transformer T is magnetically reset. Therefore, when viewed from the transformer T, only a half wave of the AC voltage contributes to power supply.

In FIG. 4, the semiconductor switch Q01 is softly switched off by the action of the semiconductor switch QR during the period from time t01 to time t05. This process is the same as the embodiment of FIG. This embodiment differs from the embodiment shown in FIG. 1 in that the voltage Vn of the transformer T has negative polarity during the period from time t04 to t'05. The process of switching on the semiconductor switch Q01 from time t06 to t07 is also the same as in the embodiment of FIG. In the embodiment of FIG. 3 as well, the effect of the resonant switch 3 is similar to that of the case of FIG. 1, and the switching loss of the semiconductor switch Q01 can be suppressed to an extremely small value. As explained in the embodiments of FIGS. 1 and 3, the semiconductor switches QR and Q1 to Q4 or Q01 do not switch a large current value, so the amount of change in current (dIab/dt or dIQ01/dt
) and the inductance of the wiring, etc., remains at a small level, and as a result, the voltage stress applied to the semiconductor switches Q1 to Q4, Q01, and QR is reduced, and malfunctions due to control device noise are less likely to occur. Become. Further, the amount of noise emitted from the DC power supply to the outside is also reduced.

FIG. 5 shows a third embodiment of the invention. In the figure, 1 is a DC power supply, 4 is a control device, RL is a load,
Q1 to Q4, QR are semiconductor switches, D1 to D
8 is a diode, DS is a snubber diode, LR
is a resonant reactor, CR is a resonant capacitor, DR is a resonant diode, Ld is a smoothing reactor, Cd is a smoothing capacitor, and T is a transformer. The connection point between the resonant capacitor CR and the semiconductor switch QR, the smoothing filter reactor Ld, and the capacitor Cd in the embodiment of FIG.
The configuration has a new snubber diode DS added between the connection point and the connection point. A series circuit of a snubber diode DS and a resonant capacitor CR is connected in parallel with the smoothing reactor Ld.

FIG. 6 is a diagram for explaining this operation. In the figure, Iab is the current flowing between points a and b, and Va
b is the voltage between points a and b, IS is the snubber diode DS
, IR is the resonant current, VR is the voltage across the resonant capacitor CR, I0 is the diode bridge D
Currents flowing through 5 to D8, q1 to q4, and qR indicate signals for turning on and off the semiconductor switches. The operation during the period from time t01 to t06 in FIG.
The case is the same as in FIG. 2 regardless of the existence of s. At time t06, current I0 is flowing through the rectifier diode, so the voltage between points PN is zero (actually, a voltage of about 2 volts corresponding to the voltage drop across the diode is output). Therefore, the charge on the resonant capacitor CR is transferred to the diode DR
The voltage VR across this terminal is almost zero. When semiconductor switches Q2 and Q3 are turned on at time t06, current Iab gradually increases, while current I0 decreases. At time t08 when current I0 becomes zero, the voltage between points PN becomes high, and current IS begins to flow through the series circuit of resonant capacitor CR and diode DS, charging capacitor CR until time t09. The voltage of this capacitor CR is diode DS
is maintained until the semiconductor switch QR is turned on next time. After time t09, the operation is the same as in the case of FIG. 2 except for the voltage VR.

Next, the effect of the diode DS will be explained. The circuit of the resonant capacitor CR and the diode DS has a surge suppression function. A diode has a drawback that if a reverse voltage is applied immediately after a forward current is applied, a current flows in the reverse direction for a certain period of time. Rectifier diode D5 ~D
Immediately after the current I0 flowing through 8 becomes zero, point X,
There is a short circuit between Y and the current flowing through reactor LR increases to more than Id. When the reverse characteristic of the diode is restored and the reverse current becomes zero, the path for the excess current flowing through the reactor LR disappears, and a surge voltage appears between points PN. However, since a path is formed by the circuit of capacitor CR, diode DS, and capacitor Cd against the excessive current that causes this surge voltage, it is possible to prevent the surge voltage from becoming excessive. It goes without saying that this diode DS can also be applied to the second embodiment shown in FIG. Furthermore, this diode DS works even without the semiconductor switch QR. That is, the resonant capacitor CR is charged by energization of the diode DS, and at this time, a surge voltage suppressing effect is produced. next,
By energizing the diode DS, the resonant capacitor CR
is discharged and returns to its initial state.

In the explanation of the embodiments so far, the resonant reactor LR was inserted in series with the secondary winding of the transformer T, but the same effect can be achieved even if the resonant reactor LR is inserted on the primary winding side of the transformer T. It acts on Also, the primary -2 of the transformer T
Since the leakage reactance between the next windings also acts equivalently as a resonant reactor, if the leakage reactance has the required size, there is no need to insert a resonant reactor LR.

FIG. 7 shows an example of the configuration of the control device 4 applied to the present invention. FIG. 8 shows an example of waveforms for explaining the operation. α is the output voltage of the DC power supply, which corresponds to the voltage of the capacitor Cd of the smoothing filter 2. Input α and the reference DC voltage to an error amplifier to obtain its output β. The triangular wave generator outputs a triangular voltage waveform γ having a frequency that causes a chopper or a DC/DC converter to perform a switching operation. The comparator compares its inputs β and γ to obtain β
The PWM signal δ is output during a period in which the level is higher than γ. The monostable multivibrator outputs a pulse signal ε having a width from the time when the PWM signal δ reaches zero level to a predetermined time. On the other hand, the flip-flop is PW
Two signals θ1 and θ2 synchronized with the time when the M signal δ rises from zero are alternately output. The OR circuit is P
The sum of the WM signal δ and the pulse signal ε is calculated to output a signal π. The AND circuit 1 multiplies the signal θ1 and π and outputs the signal ω1. Similarly, as the output of AND2, the signal ω
Get 2. To operate the one-stone forward converter of Figure 3, signals π and ε are connected to semiconductor switches Q01 and Q, respectively.
Give to R. To operate the DC/DC converters shown in FIGS. 1 and 5, a signal ω1 is applied to the semiconductor switch pair Q1 and Q4, and a signal ω2 is applied to Q2 and Q3. Also, a signal ε is given to QR.

[0023]

[Effects of the Invention] According to the present invention, the switching loss of the semiconductor switch can be suppressed to a low level, making it possible to perform switching at a higher frequency.This makes it easier to downsize transformers, filters, etc., and makes it possible to downsize DC power supplies. development is progressing. Additionally, if the switching frequency is not high, the cooling system for the semiconductor switch may be simple;
In this respect, it is possible to contribute to miniaturization of the device and increase the efficiency of the device. Since semiconductor switches do not forcibly switch large currents, the spike voltages and noise that occur with switching are reduced, making the device more reliable and reducing noise pollution to the outside. Furthermore, in the invention shown in FIG. 5, even if a surge voltage appears between the output terminals of the diode bridge, the snubber diode can prevent this surge voltage from becoming excessive.

[Brief explanation of the drawing]

FIG. 1 shows a first embodiment of a DC power supply device of the present invention.

FIG. 2 is a diagram for explaining the operation of the first embodiment.

FIG. 3 shows a second embodiment of the invention.

FIG. 4 is a diagram for explaining the operation of the second embodiment.

FIG. 5 shows a third embodiment of the invention.

FIG. 6 is a diagram for explaining the operation of the third embodiment.

FIG. 7 shows an example of a control device constituting the present invention.

FIG. 8 is a diagram for explaining the operation of the control device.

FIG. 9 shows a conventional example.

FIG. 10 is a diagram illustrating the operation of a conventional example.

[Explanation of symbols]

1 DC power supply 2 Smoothing filter 3 Resonance switch 4 Control device E DC power supply voltage T Transformer RL Load Q1 to Q4, QR Semiconductor switch D1 to D
8 Diode DS Snubber diode LR Resonant reactor Ld Smoothing reactor Cd Smoothing capacitor CR Resonant capacitor

Claims (3)

[Claims] Claim 1: A DC power supply and a primary winding of a transformer are connected through a semiconductor switch, and a smoothing filter constituted by a series circuit of a reactor and a capacitor is connected to the secondary winding of the transformer through a rectifier diode. In the DC power supply device that supplies power from the capacitor to the load, a resonant switch consisting of a parallel circuit of a diode and a semiconductor switch, and a resonant capacitor connected in series to the parallel circuit is provided in parallel on the DC output side of the rectifier diode. A DC power supply device characterized by: 2. The direct current according to claim 1, wherein a snubber diode is connected between a connection point between a resonant capacitor and a semiconductor switch constituting a resonant switch and a connection point between a reactor and a capacitor of the smoothing filter. power supply. 3. A DC power source and a primary winding of a transformer are connected via a semiconductor switch, and a smoothing filter configured of a series circuit of a reactor and a capacitor is connected to the secondary winding of the transformer via a rectifier diode. In a DC power supply device that supplies power from the capacitor to a load, a series circuit of a diode and a resonant capacitor is provided in parallel with the DC output of the rectifier diode, and a connection point between the resonant capacitor and the diode, a smoothing filter reactor, and the capacitor are connected. A DC power supply device characterized in that the DC power supply device is connected to a connection point via a snubber diode.