GB2371928A

GB2371928A - DC-DC power converter

Info

Publication number: GB2371928A
Application number: GB0126316A
Authority: GB
Inventors: John Stephens
Original assignee: Advanced Power Conversion Ltd
Current assignee: Advanced Power Conversion Ltd
Priority date: 2001-02-02
Filing date: 2001-11-02
Publication date: 2002-08-07
Also published as: AU2002228291A1; GB0102675D0; GB0126316D0; WO2002061928A3; WO2002061928A2

Abstract

A DC-DC converter comprises a transformer, the primary coil X<SB>p</SB> of which is connected to a primary circuit, and the secondary coil X<SB>s</SB> of which is connected to a secondary circuit. The primary circuit, includes at least a first primary capacitor C<SB>1</SB> and a first primary choke winding L<SB>1p</SB>, the primary capacitor being charged and discharged in an cyclic manner, such that an AC current flows through the primary coil when a DC source is connected to the primary circuit. The secondary circuit, includes at least a first secondary choke winding L<SB>1s</SB>, and rectification means RS<SB>3</SB>,RS<SB>4</SB> such that an AC current induced in the secondary coil is substantially converted to a DC output of the secondary circuit, characterised in that the first primary choke winding being connected in series with an first primary external inductor L<SB>1p</SB>(ext) whilst not being substantially coupled to the first primary external inductor, and/or the first secondary choke winding being connected in series with a first secondary inductor L<SB>1s</SB>(ext) whilst not being substantially coupled to the first secondary external inductor. The first primary choke winding and the first secondary choke winding may be inductively coupled. Switching elements RS3,RS4,RS11,RS12,RS21,RS22 may be MOSFETS.

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 transformer, the primary coil of which is connected to a primary circuit, and the secondary coil of which is connected to a secondary circuit, the primary circuit, including at least a first primary capacitor and a first primary choke winding, the primary capacitor being charged and discharged in an cyclic manner, such that an AC current flows through the primary coil when a DC source is connected to the primary circuit, the secondary circuit, including at least a first secondary choke winding, and rectification means, such that an AC current induced in the secondary coil is substantially converted to a DC output of the secondary circuit, characterised in that the first primary choke winding being connected in series with an first primary external inductor whilst not being substantially coupled to the first primary external inductor, and/or the first secondary choke winding being connected in series with a first secondary inductor whilst not being substantially coupled to the first secondary external inductor.

Preferably the first primary choke winding and the first secondary choke winding are inductively coupled.

Preferably the primary circuit includes a second primary capacitor and a second primary choke winding, the second primary capacitor being charged and discharged in an cyclic manner alternately with the first primary choke, and the secondary circuit including at least a second secondary choke winding.

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 and 6 show other embodiments of converters ; Figure 7 shows an alternative switching timing of the converter; and Figure 8 shows another 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'E'shaped core pieces, each core piece having three limbs. The core pieces are placed together to form three limbs. Primary and secondary transformer windings xp and Xs are provided on the centre limb of the ferrite core. Windings L, p and LIs are provided on an outer limb to form primary and secondary chokes, and windings L2p and L25 on the other outer limb form a second pair or primary and secondary chokes. Other core shapes, such as'I'shaped pieces, may be used.

A capacitor C, is provided between the primary transformer winding xp and the first primary choke winding L1p. Similarly, a capacitor C2 is provided between the primary transformer winding xp and the second primary choke winding L2p. Two switches RS and RS21 are coupled between the capacitor C, and the first primary choke winding L, p, and capacitor C2 and the second primary choke winding L2p respectively.

Similarly, two switches RS12 and RS22 are coupled between the capacitor C, and the primary transformer winding xp, and capacitor C2 and the primary transformer winding xp. The switches RS11 and RS, 2 and the capacitor C, form a first switching circuit for the first primary choke winding LIp, and the switches RS2J and RS22 and the capacitor C2 form a second switching circuit for the second primary choke winding L2p An input voltage is applied to the first primary choke winding Lip and the first switching circuit, and to the second primary choke winding L2p and the second switching circuit.

In the secondary circuit, the secondary transformer winding Xs is connected between the first secondary choke winding Lis and the first secondary choke winding L2,. Two switches RS3 and RS4 are coupled to points between the secondary transformer winding Xs and the first secondary choke winding Lis, and between the secondary transformer winding Xs and the second secondary choke winding L2s. A capacitor Co is coupled across the output for smoothing.

Referring to figure 2, the basic cycle of the circuit has four separate

the second phases. In this figure, the first primary choke is labelled L the second primary choke is labelled Lxfmr2, the primary transistor winding is labelled L3, the secondary transistor winding is labelled L, , first secondary choke is labelled Lxfmrll, the second secondary choke is labelled Lmr22-It will also be noticed that for the purposes of explaining the basic mode of operation, the pair of switches RSII and RSI2, has been illustrated as a single switch having two contact positions RS11 and RS12. Switch pair RS21 and RS22 have been simplified in a corresponding manner. A capacitor C, is added across the voltage source (here a battery) to smooth the input.

In an initial state (say before to), switches RS12 and RS22 are closed in the primary circuit, while RS and RS2, are open. Capacitor CI is charged through the first primary choke winding Lisp, and similarly capacitor C2 is charged through the second primary choke winding L2p at the input voltage. Capacitors CI and C2 are sufficiently large to smooth the ripple voltage caused by switching, and the choke windings Lip and L2p are sufficiently large to smooth the ripple current.

Also included are external inductors Lla and Lib in series with Lisp, and L2p respectively. When the inductors L1p, L1s, L2p and L2s are tightly coupled, a ripple in the input and output currents may result. Providing loosely coupled external inductors introduces an additional element which negates or'steers'the unwanted ripple current, as further discussed below.

As well as providing external inductors on both primary and secondary circuits, external inductors can also be provided solely on the primary circuit, or solely on the secondary circuit.

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 ; ;, RS, RS, and Razz, the voltage across the switches RS, RS, RS, and RS22, the current across the first and second primary choke windings Lip and Lip, the current through the capacitors C, and C2, the voltage across the primary transformer winding xp, the magnetic current across the primary transformer winding xp, the total current across the primary transformer winding xp, the voltage across the secondary transformer winding Xs, the current through the diodes D, and Dz, (which are substantially equivalent to the switches RS3 and RS4 of figure 1) the current across the first and second secondary choke windings LIs and L1s, and the current through the smoothing capacitor Co.

Referring back to Figure 2, at a time to, switch PS, ; is closed and RS is opened. The voltage across C is applied upon the primary transformer winding xp, the secondary voltage at x, reverse biases D ; and causes the current to increase through xi. te current in D2 increases to the sum of both the currents flowing in Lxfmr11 and L. The output voltage is dependent upon the turn ratio of the primary and secondary transformer

windings xp and xs, the secondary choke arrangement halving the output voltage, half the current flowing through each inductor.

At a time tl, switch RS I I is opened and RS12 is closed. The primary transformer winding xp is clamped by RS22 and RS12 to 0 volts, the stored energy in the primary transformer winding xp circulating as current. The secondary transformer winding Xs is clamped by the primary transformer winding. Energy stored in the first secondary choke winding circulates as current through D,. Current through the first primary choke Lip decreases and capacitor C, recharges. The current through Ls decreases.

At a time t2, switch RS22 is opened and RS21 is closed. The charge on capacitor C2 discharges through the primary transformer winding xp, taking the lower connection negative and causing a current to flow through D, and Xg. 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

p'12 to 0 Volts, transformer winding xp is clamped by RS22 and RS to 0 volts, the stored energy in the primary transformer winding xp circulating as current. The secondary transformer winding x, 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 t4, the circuit switches as described from to, and the cycle is repeated indefinitely.

It can be shown that the output voltage, VO, Vo =N,/Np. V,. D/ (l-D) where V, is the input voltage,. N/Np 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 5us period, as shown in Figure 4.

Initially, only switch RS12 is closed, whilst all the other switches are open.

After 0. 1 lus, switch RS22 is closed. After 0. 7, us from the beginning of the cycle switch RS, 2 is opened. At 0. 811s from the beginning of the cycle, RS is closed. 2. 5jus through the cycle, RS is opened. At 2. 6us from the beginning of the cycle, switch RS12 is closed. At 3. 2, us from the beginning of the cycle, switch RS22 is opened, and at 3. 3As from the beginning of the cycle, switch RS2, is closed. Switch RS21 is opened 5, us 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, and D2 follow the timings of RS12 and RS22 respectively. Switches, such as MOSFETs could though be used instead or diodes.

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 and/or adding external inductances, 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.

The ferrite core indicated in figure 1 as a dotted line, and as previously mentioned, is formed from two'E'or'I'shaped core pieces.

Referring to figure 5, four external inductors Llp (exi), Lext), L (ext), and L2s(ext) are provided in series with L, p, Lls, L2p and 12s respectively.

When the inductors LIp, Lis, Lzp and L2s are tightly coupled, a ripple in the input and output currents may result. Providing loosely coupled external inductors introduces an additional element which negates or'steers'the unwanted ripple current. As previously shown in figure 2, external inductors may be provided solely on the primary circuit (i. e. Llp (ext) and L2p (eut) only). Equally, they may be provided solely on the secondary circuit (i. e. Ll, (ext) and Lzs (ext) only).

It will be realised of course that different inductors may be coupled by different degrees. Where inductors (other than the primary and secondary transformer windings) are magnetically coupled to a significant degree, external inductors loosely coupled or not substantially coupled may be introduced in series, so that the input and/or output ripple currents are steered to reduce the ripple currents.

The need for external inductors, and what value inductance should be used, will depend upon the magnetic properties of the circuit. In general, magnetically integrated circuits which are tightly coupled and have a low leakage inductance will benefit more than partially integrated or discrete circuits which will be loosely coupled and have a significant leakage inductance. If, for example, the transformer and choke windings are provided on a printed circuit board, the circuit will typically be tightly coupled (i. e. have a low leakage inductance), and the provision of external inductors (not here shown) will reduce the ripple current. The external inductors may be provided either in series with both primary choke windings, or in series with both secondary choke windings, or in series with both.

Referring to Figure 6, capacitors Crsll and Cri may be provided in parallel with switches RS11 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 LZVRT- The capacitances Crs11 and Cri are coupled with the primary transformer winding's reactance to establish a resonant circuit such that the switching is effected at zero volts.

When RS11 is closed, the charge accumulated on Crsl discharges through RS, and Cri similarly discharges on RS12's closing. In this manner, a waveform is obtained that counteracts the effect of the parasitic

inductance LZVRT 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 (CV=IT). The capacitance, rather than being provided as a discrete component, may be an integral parasitic feature of the MOSFET (coif it is external to the MOSFET i. e. additional capacitors as shown, then there are greatly reduced turn-OFF losses in RS and RS2.

The inductance Lzvrt and the capacitors Crs11 and Crs21 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 ZVRT (Zero Volt Resonant Transition) switching noise (and the losses it causes) of RS and RS21 can be reduced. It is all dependant upon the rate of change of voltage across the MOSFETs, this being controlled by the

capacitors Cru 1 1 and Crs21. External chokes may be provided coupled either to the primary chokes, the secondary chokes, or to both.

Other switching regimes may be followed. Such a further switching regime is shown in Figure 7. As is the previous example, the switching cycle is over a 50 s period. Initially, only switches D, and D2 are closed, all the other switches being open. After 0.1 fis from the beginning of the cycle, switches RS22 is closed. After 0. 8 s from the beginning of the cycle

switch RS, t is closed whilst D, is opened. At 2. 5 gus from the beginning of the cycle, switches RS and RS22 are opened whilst D, is closed. 2. 6us through the cycle, switch RS2, is closed. At 3. 3 us from the beginning of the cycle, switch Ris, 2 is closed and D2 is opened. At the end of the cycle, i. e. 5, us from the beginning of the cycle, switch RS, is opened and RS21 and D2 are closed. The cycle then repeats.

In this timing regime, it can be seen that the switches RS 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 similar secondary circuits, as shown in Figure 8, each secondary circuit magnetically coupled to the primary circuit. External inductors may be fitted in series with the chokes of the primary circuit, and/or either or both the secondary circuits.

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,), RS D, and D2 could be n-channel MOSFETs, while RS21 and RS22 are p-channel

MOSFETs. Alternatively, a p-channel MOSFET with a Shottky diode coupled across it could be used for the switches RS2, and RS22. Diodes D ; and D2 could conveniently be Shottky 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

Claims 1 A DC-DC converter comprising: a transformer, the primary coil of which is connected to a primary circuit, and the secondary coil of which is connected to a secondary circuit, the primary circuit, including at least a first primary capacitor and a first primary choke winding, the primary capacitor being charged and discharged in an cyclic manner, such that an AC current flows through the primary coil when a DC source is connected to the primary circuit, the secondary circuit, including at least a first secondary choke winding, and rectification means, such that an AC current induced in the secondary coil is substantially converted to a DC output of the secondary circuit, characterised in that the first primary choke winding being connected in series with an first primary external inductor whilst not being substantially coupled to the first primary external inductor, and/or the first secondary choke winding being connected in series with a first secondary inductor whilst not being substantially coupled to the first secondary external inductor.
2. A converter according to claim I characterised in that the first primary choke winding and the first secondary choke winding are inductively coupled.
3. A converter according to claim 1 characterised in that the primary circuit includes a second primary capacitor and a second primary choke winding, the second primary capacitor being charged and discharged in an cyclic manner alternately with the first primary choke, and the secondary circuit including at least a second secondary choke winding.
4. A converter according to claim 3 characterised in that the second primary choke winding is inductively coupled to the first primary choke winding.
5. A converter according to either claim 3 or 4 characterised in that the .. second primary choke winding is connected in series with a second primary external inductor whilst not being substantially coupled to the second primary external inductor.
6. A converter according to any of claims 2 to 5 characterised in that the second secondary choke winding is connected in series with a second secondary external inductor whilst not being substantially coupled to the second secondary inductor.
7. A converter according to claim 6 characterised in that the second secondary choke is inductively coupled to the first secondary choke.
8. A converter according to either claim 6 or 7 characterised in that the second secondary choke is inductively coupled to the second primary choke.
9. A converter according to any of claims 2 to 8 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.
10. A primary circuit according to any previous claim, characterised in that the transformer coils and the choke windings are provided by a conductive paths deposited on a planar or laminate structure, and the external inductor or inductors are provided by discrete components.
11. A converter as herein described and illustrated.
12. Any novel and inventive feature or combination of features specifically disclosed herein within the meaning of Article 4H of the International Convention (Paris Convention).