WO2002061925A2

WO2002061925A2 - A converter

Info

Publication number: WO2002061925A2
Application number: PCT/IB2002/000394
Authority: WO
Inventors: John Stephens
Original assignee: Advanced Power Conversion Plc
Priority date: 2001-02-02
Filing date: 2002-02-01
Publication date: 2002-08-08
Also published as: AU2002234816A1; GB2371929A; GB0102674D0; GB0126352D0; WO2002061925A3

Abstract

A DC-DC converter comprises a primary circuit fed by a DC power source. The primary circuit comprises a transformer primary winding having a first end coupled to one side of the DC power source through a capacitor and inductor, the capacitor being coupled to the second side of the DC power source through a switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor; a transformer coupling the primary and secondary circuits together; a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately. The control circuitry for the switching means causes a transient delay in the closing of one switch relative to the opening of the other switch of the switching means.

Description

A Converter

The present invention relates to a converter, typically for a power supply for supplying a continuous output current, from a continuous input current, with particular applications, amongst others, as power supplies for examples in automotive or telecoms applications.

Transformers used in electrical and electronic applications for

'transforming' an input voltage to a higher or lower voltage (and often referred to as "Buck" and "Boost" converters respectively) are well known to persons skilled in the art. A problem with known transformers is to provide assemblies which operate with both continuous input and output currents. This is possible with a series of boost and buck converters, but a simple cascade of these two has an increased component count and is additionally complex to drive the devices.

A known DC - DC converter is described in US 5 886 882 (Rodolpho), which features primary and secondary transformer windings, together with two pairs of primary and secondary choke windings, wound upon a three limbed core. A switching circuit is coupled between the first primary choke and the primary transformer winding, and a similar switching circuit is coupled between the first secondary choke and the primary transformer winding. Each switching circuit comprises a capacitor, diode and a MOSFET. The switching circuits are switched on and off in a cyclic manner (the MOSFETs being driven by two interleaved square pulse trains) to provide a continuous output current from a continuous input current in a push-pull manner. It is an object of the present invention to provide a DC to DC converter and method of driving it to efficiently produce a continuous output current for a continuous input current.

According to the present invention there is provided a DC-DC converter comprising:

a primary circuit fed by a DC power source;

the primary circuit comprising a transformer primary winding having a first end coupled to one side of the DC power source through a capacitor and inductor, the capacitor being coupled to the second side of the DC power source through a switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor;

a transformer coupling the primary and secondary circuits together;

a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and

control circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately;

characterised in that the control circuitry for the switching means causes a transient delay in the closing of one switch relative to the opening of the other switch of the switching means. Preferably, the second end of the transformer primary winding is connected to the DC power source through a second capacitor and second inductor, the second capacitor being coupled to the second side of the DC power source through a second switching means comprising a second pair of switches connected to the inductor side and transformer primary winding side of the capacitor;

Preferably, there is provided a delay between one capacitor discharging through the primary coil, and the other capacitor discharging through the primary coil.

According to another aspect of the invention, there is provided a DC-DC converter comprising:

a primary circuit fed by a DC power source;

the primary circuit comprising a transformer primary winding having each end coupled to the same side of the DC power source through a respective capacitor and inductor, each capacitor being coupled to the second side of the DC power source through respective switching means comprising a pair of switches connected to the inductor side and transformer primary winding side of the capacitor;

a transformer coupling the primary and secondary circuits together;

a secondary circuit including a transformer secondary winding and rectifying means for producing a DC output; and control circuitry operating the switches of each switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of each capacitor alternately, such that each capacitor discharges through the primary winding alternately;

characterised in that the control circuitry operates the at least one of the switches when there is substantially no voltage across it.

a primary circuit fed by a DC power source;

a transformer coupling the primary and secondary circuits together;

control circuitry operating the switches of each switching means to connect the second side of the DC power source to the inductor side and transformer primary winding side of each capacitor alternately, such that each capacitor discharges through the primary winding alternately; characterised in that for one or both switch means, the switch connected to the transformer primary winding side of the capacitor is kept open for approximately twice the period over a complete switching cycle that the switch connected to the inductor side of the capacitor is kept open for.

A converter according to the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a circuit diagram of a converter;

Figure 2 shows the current flow during the basic phases of a cycle in the converter;

Figure 3 shows the voltages and currents across particular components due to the basic phases;

Figure 4 shows the switching timings of the converter;

Figures 5 to 9 shows other embodiments of the converter;

Figure 10 shows alternative switching timings of the converter; and

Figure 11 shows a further embodiment of the converter;

Referring to Figure 1, the converter comprises an input, an output, and a transformer assembly having a ferrite core. The core is formed from two Ε' shaped core pieces, each core piece having three limbs. The core pieces are placed together to form three limbs. Primary and secondary transformer windings x_p and x_s are provided on the centre limb of the ferrite core. Windings L_lp and L_ls are provided on an outer limb to form primary and secondary chokes, and windings L_2p and L_2s on the other outer limb form a second pair or primary and secondary chokes. Other core shapes, such as T shaped pieces, may be used.

A capacitor is provided between the primary transformer winding x_p and the first primary choke winding L_lp. Similarly, a capacitor C₂ is provided between the primary transformer winding x_p and the second primary choke winding L_2p. Two switches RS_n and RS₂₁ are coupled between the capacitor C_l and the first primary choke winding L_lp, and capacitor C₂ and the second primary choke winding L_2p respectively. Similarly, two switches RS₁₂ and RS₂₂ are coupled between the capacitor C₁ and the primary transformer winding x_p, and capacitor C₂ and the primary transformer winding x_p. The switches RS_π and RS₁₂ and the capacitor form a first switching circuit for the first primary choke winding L_lp, and the switches RS₂₁ and RS₂₂ and the capacitor C₂ form a second switching circuit for the second primary choke winding L_2p

An input voltage is applied to the first primary choke winding L_lp and the first switching circuit, and to the second primary choke winding L_2p and the second switching circuit.

In the secondary circuit, the secondary transformer winding x_s is connected between the first secondary choke winding L_ls and the first secondary choke winding L_2s. Two switches RS₃ and RS₄ are coupled to points between the secondary transformer winding x_s and the first secondary choke winding L_ls, and between the secondary transformer winding x_s and the second secondary choke winding L_2s. A capacitor C₀ is coupled across the output for smoothing.

Referring to figure 2, the basic cycle of the circuit has four separate phases. In this figure, the first primary choke is labelled L_xfmrl, the second primary choke is labelled L_xfmr2, the primary transistor winding is labelled L_xfinr3, the secondary transistor winding is labelled L_xflnr33, first secondary choke is labelled L_xfinrll, the second secondary choke is labelled L_xftnr22. It will also be noticed that for the purposes of explaining the basic mode of operation, the pair of switches RS_π and RS₁₂, has been illustrated as a single switch having two contact positions RS_π and RS₁₂. Switch pair RS₂₁ and RS₂₂ have been simplified in a corresponding manner. A capacitor is added across the voltage source (here a battery) to smooth the input.

In an initial state (say before t₀), switches RS₁₂ and RS₂₂ are closed in the primary circuit, while RS_π and RS₂₁ are open. Capacitor is charged through the first primary choke winding L_lp, and similarly capacitor C₂ is charged through the second primary choke winding L_2p at the input voltage. Capacitors and C₂ are sufficiently large to smooth the ripple voltage caused by switching, and the choke windings L_lp and L_2p are sufficiently large to smooth the ripple current.

Figure 3 shows the currents and or voltages across various of the components in the circuit through two switching cycles. Figure 3 shows respectively the current across the switches RS_n, RS₁₂, RS₂₁, and RS₂₂, the voltage across the switches RS_π, RS₁₂, RS₂₁, and RS₂₂, the current across the first and second primary choke windings L_lp and L_lp, the current through the capacitors and C₂, the voltage across the primary transformer winding x_p, the magnetic current across the primary transformer winding x_p, the total current across the primary transformer winding x_p, the voltage across the secondary transformer winding x_s, the current through the diodes Dj and D₂, (which are substantially equivalent to the switches RS₃ and RS₄ of figure 1) the current across the first and second secondary choke windings L_ls and L_ls, and the current through the smoothing capacitor C₀.

Referring back to Figure 2, at a time t₀, switch RS_π is closed and RS₁₂ is opened. The voltage across is applied upon the primary transformer winding x_p, the secondary voltage at x_s reverse biases O_l and causes the current to increase through x_s. The current in D₂ increases to the sum of both the currents flowing in L_xfinrll and L-. The output voltage is dependent upon the turn ratio of the primary and secondary transformer windings x_p and x_s, the secondary choke arrangement halving the output voltage, half the current flowing through each inductor.

At a time t_l5 switch RS_π is opened and RS₁₂ is closed. The primary transformer winding x_p is clamped by RS₂₂ and RS₁₂ to 0 volts, the stored energy in the primary transformer winding x_p circulating as current. The secondary transformer winding x_s is clamped by the primary transformer winding. Energy stored in the first secondary choke winding circulates as current through D_x. Current through the first primary choke L_lp decreases and capacitor C_x recharges. The current through L_s decreases.

At a time t₂, switch RS₂₂ is opened and RS₂₁ is closed. The charge on capacitor C₂ discharges through the primary transformer winding x_p, taking the lower connection negative and causing a current to flow through Dj and x_s. As previously, the output voltage is dependent upon the turn ratio of the transformer windings. At a time t₃, switch RS₂₁ is opened and RS₂₂ is closed. The primary transformer winding x_p is clamped by RS₂₂ and RS₁₂ to 0 volts, the stored energy in the primary transformer winding x_p circulating as current. The secondary transformer winding x_s is clamped by the primary transformer winding. Energy stored in the second secondary choke winding circulates as current through D₂. This phase resets the transformer. At t , the circuit switches as described from t₀, and the cycle is repeated indefinitely.

It can be shown that the output voltage, V₀,

V₀ = NJ _P . V, . D/(1-D)

where V; is the input voltage,. N_s/N_p is the transformer turn ratio, and D is the duty cycle of the switching circuits.

The switches are operated by a control circuit (not here shown). A typical switching cycle would be over a 5μs period, as shown in Figure 4. Initially, only switch RS₁₂ is closed, whilst all the other switches are open. After 0.1 μs, switch RS₂₂ is closed. After 0.7μs from the beginning of the cycle switch RS₁₂ is opened. At 0.8μs from the beginning of the cycle, RS_π is closed. 2.5μs through the cycle, RS_π is opened. At 2.6μs from the beginning of the cycle, switch RS₁₂ is closed. At 3.2μs from the beginning of the cycle, switch RS₂₂ is opened, and at 3.3μs from the beginning of the cycle, switch RS₂₁ is closed. Switch RS₂₁ is opened 5μs from the beginning of the cycle, which marks the start of a new cycle. It will also be seen that during this time, diodes D_t and D₂ follow the timings of RS₁₂ and RS₂₂ respectively. Switches, such as MOSFETs could though be used instead or diodes.

When each switching circuit is switched, it will be seen that there is a delay between the first switch opening and the second switch closing, (i.e. between RS₁₂ opening and RS₂₂ closing, between RS₂₂ closing and RS₁₂ closing, between RS₂₂ opening and RS₂₁ closing, and between RS₂₁ opening and RS₂₂ closing). Under this timing sequence, the delay is 0.1 μs, but the optimum delay is dependant upon the components in the circuit, throughput power, supply voltage, operational switching frequency and other effects. Typical delays range from 50ns to 100ns.

The ferrite core indicated in figure 1 as a dotted line, and as previously mentioned, is formed from two Ε' or T shaped core pieces. The ferrite cores need not be in this integral form however. Referring to figure 5, the windings L_lp, L_ls, L_2p and L_2s may be provided as discrete magnetic components (Windings x_p and x_s sharing a single magnetic core in this and every other embodiment).

Referring to Figure 6, the first and second primary choke windings L_lp and L_2p may be magnetically coupled to the transformer windings x_p and x_s. The first and second secondary choke windings L_ls and L_2s are each magnetically discrete components. Conveniently, the first and second primary choke windings L_lp and L_2p are wound upon the outer limbs of the core, while the first and second secondary choke windings L_ls and L_2s are provided as discrete inductors. Referring to Figure 7, the first and second secondary choke windings L_ls and L_2s are magnetically coupled to the transformer windings x_p and x_s, the first and second primary choke windings L_lp and L_2p being magnetically discrete components. In a similar manner to the previous embodiment the first and second secondary choke windings L_ls and L_2s are conveniently wound upon the outer limbs of the core, while the first and second primary choke windings L_lp and L_2p are provided as discrete inductors.

In an alternative circuit, shown in figure 8, the first primary choke winding L_lp is magnetically coupled to the first secondary choke winding L_ls in a discrete magnetic component, and the second primary choke winding L_2p is likewise magnetically coupled to the second secondary choke winding L_2s.

It will be realised of course that different inductors may be coupled by different degrees. Rather than all the inductors (with the exception of the transformed inductors, which are always substantially tightly coupled) being discrete components with no coupling between them, some inductors may be loosely coupled.

A fully or partially integrated form of circuitry, where the primary and secondary choke windings are coupled, allows the input and output ripple currents to be steered so that the net output is relatively free from ripple.

The switching of a switching circuit associated with a particular winding applies an AC waveform, via the decoupling capacitor, to a any second or further winding which is magnetically coupled to the first. By applying an AC waveform to the second winding matching the switching waveform of the first winding, the current flow into the first winding, caused by the switching action, can be halved. Also, by changing the turns ratio between the first and second windings, it becomes possible to further reduce the ripple current in the first winding, to the extent that the switching frequency ripple current can be reduce close to zero. This technique has the effect of apparently increasing the inductance of the first winding to a value significantly greater than the actual electronic value.

Referring to Figure 9, capacitors C_rsll and C_rs21 may be provided in parallel with switches RS_π and RS₂₁. The primary transformer winding is in reality not completely coupled to the secondary winding, but includes a component of pure inductor which is represented here as L_ZVRT.

The capacitances C_rsll and C_rs21 are coupled with the primary transformer winding's reactance to establish a resonant circuit such that the switching is effected at zero volts.

When RS_n is closed, the charge accumulated on C_rsll discharges through RS_π, and C_rs21 similarly discharges on RS₁₂'s closing. In this manner, a waveform is obtained that counteracts the effect of the parasitic inductance L_ZVRT and reduces the losses otherwise attributable to it.

Rs21 and Rs22 are, by circuit operation, switched-ON with zero volt across them. On opening, the current flow is through the capacitors due to the capacitors charging-up (CN=IT). The capacitance, rather than being provided as a discrete component, may be an integral parasitic feature of the MOSFET (C_oss). If it is external to the MOSFET i.e. additional capacitors as shown, then there are greatly reduced turn-OFF losses in RS_n and RS₂₁.

The inductance _zvrt and the capacitors C_rsll and C_rs21 form a resonant tank swinging the voltage across the switch to zero at which point the MOSFET, Rsl 1 or Rs21 as the case may be, are switched-ON.

By switching the MOSFETs at the correct time and utilising ZNRT (Zero Volt Resonant Transition) switching noise (and the losses it causes) of RS_π and RS₂₁ can be reduced. It is dependant upon the rate of change of voltage across the MOSFETs, this being controlled by the capacitors Crsl 1 and Crs21.

Other switching regimes may be followed. Such a further switching regime is shown in Figure 10. As is the previous example, the switching cycle is over a 5μs period. Initially, only switches Dj and Dj (referring also to the circuit in Figure 2) are closed, all the other switches being open. After 0.1 μs from the beginning of the cycle, switches RS₂₂ is closed. After 0.8μs from the beginning of the cycle switch RS_π is closed whilst Dj is opened. At 2.5μs from the beginning of the cycle, switches RS_π and RS₂₂ are opened whilst Dj is closed. 2.6μs through the cycle, switch RS₂₁ is closed. At 3.3μs from the beginning of the cycle, switch RS₂₁ is closed and D₂ is opened. At the end of the cycle, i.e. 5μs from the beginning of the cycle, switches RS₁₂ and RS₂₁ are opened and D₂ is closed. The cycle then repeats.

In this timing regime, it can be seen that the switches RS_π and RS₂₂ are kept open for a longer period than in the previous timing regime. The switches therefore are not switched at zero volts, and cannot be switched in a resonant manner to reduce the inductive losses of the transformer winding. When in the open position however, the switches conduct no current and therefore will not dissipate energy, so the circuit is made more efficient.

The primary circuit may drive two or more secondary circuits. Figure 11 shows two secondary circuits (the second secondary circuit having similar components as the secondary circuit already described) magnetically coupled to the primary circuit. The windings and capacitance's may of course be different, so that the two secondary circuits give different output voltages.

The switches described here have been MOSFETs, but other switching devices could substituted. Conveniently, RS_π, RS₁₂, D_t and D₂ could be n-channel MOSFETs, while RS₂₁ and RS₂₂ are p-channel

MOSFETs. Alternatively, a p-channel MOSFET with a Schottky diode coupled across it could be used for the switches RS₂₁ and RS₂₂. RS₂₁ and RS₂₂ could also be n-channel MOSFETs with a Schottky diode coupled across it. Diodes Dj and D₂ could conveniently be Schottky diodes. Other suitable transistors or other switching devices will be apparent to one skilled in the art.

The principles disclosed herein could equally be applied to other DC-DC converter topographies, such as circuits having only one discharging capacitor in the primary circuit, and a correspondingly simplified secondary circuit.

Claims

1. A DC-DC converter comprising:

a primary circuit fed by a DC power source;

a transformer coupling the primary and secondary circuits together;

characterised in that the control circuitry for the switching means causes a transient delay in the closing of one switch relative to the opening of the other switch of the switching means.

2. A converter according to claim 1, wherein the second end of the transformer primary winding is connected to the DC power source through a second capacitor and second inductor, the second capacitor being coupled to the second side of the DC power source through a second switching means comprising a second pair of switches connected to the inductor side and transformer primary winding side of the capacitor;

3. A converter according to previous claim 2 characterised in that there is provided a delay between one capacitor discharging through the primary coil, and the other capacitor discharging through the primary coil.

4. A converter according to claim 3 characterised in that period of the delay is approximately the same as the period over which one of the capacitors is discharged.

5. A converter according to any previous claim characterised in that the control circuitry operates at least one of the switches when there is substantially no voltage across it.

6. A converter according to any previous claim characterised in that for one or both switch means, the switch connected to the transformer primary winding side of the capacitor is kept open for approximately twice the period over a complete switching cycle that the switch connected to the inductor side of the capacitor is kept open for.

7. A converter according to any previous claim characterised in that the control circuitry operates the switches such that the switch coupled when the voltage across it is substantially zero.

8. A converter according to any previous claim characterised in that the transient delay is approximately lμs.

9. A DC-DC converter comprising:

a primary circuit fed by a DC power source;

a transformer coupling the primary and secondary circuits together;

10. A DC-DC converter comprising:

a primary circuit fed by a DC power source;

a transformer coupling the primary and secondary circuits together;

a secondary circuit including a fransformer secondary winding and rectifying means for producing a DC output; and

confrol circuitry operating the switches of the switching means to connect the second side of the DC power source to the inductor side and fransformer primary winding side of the capacitor alternately, such that the capacitor discharges through the primary winding alternately;

characterised in that for one or both switch means, the switch connected to the fransformer primary winding side of the capacitor is kept open for approximately twice the period over a complete switching cycle that the switch connected to the inductor side of the capacitor is kept open for.

11. A primary circuit according to any previous claim.

12. A converter as herein described and illustrated.

13. Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).