GB2126806A - Chopper circuit - Google Patents

Abstract

Current surges in the series element (T1) of a chopper circuit supplying an inductive load (L) are reduced by the incorporation of a saturating transformer (TF) which has two counterwound windings (NL, NF) disposed on a high permeability core in series with the load (L), with the number of turns in the load winding (NL) exceeding that of the source winding (Ns), and with one side of a freewheel diode (D1) connected to the junction of the two windings. The arrangement is such that when the series element (T1) is non-conductive the load current flowing in the load winding (NL) maintains the core in saturation but when the series element becomes conductive once again and reverse current flows in the freewheel diode (D1) the transformer is taken into a high permeability region to limit the level of the latter current. <IMAGE>

Description

SPECIFICATION Chopper circuit The present invention relates to chopper circuits of the type used in DC controllers and AC inverters. In particular, the invention is concerned with chopper circuits of the type whose output end includes a switchable element, such as a transistor or thyristor, disposed in series with an inductive load, the switching of the series element determining the average d.c. level applied to the load from a fixed or variable d.c. supply.

In such an arrangement, a return path has to be provided for the current which continues to flow in the inductive load when the series element is in its off condition. This is achieved by means of a freewheel diode disposed in parallel with the load.

Thus, when the series element is switched off, the load current, which was flowing from the d.c.

supply through the series element to the load, then flows through the freewheel diode and back to the load.

The basic known arrangement has several problems associated with it in practice. During the off period of the series element, the current in the free wheel diode tends to fall but it can still be flowing, i.e. the free wheel diode is still forward biassed, when the series element is switched on again. When the series element switches on it takes over current from the diode and the current in the latter rapidly reduces to zero. However, when the diode current is zero, the diode does not recover its reverse blocking capability immediately but conducts reverse current for a short time due to diode recombination caused by majority carriers being swept out of the p-n junction region. The diode then recovers and quickly switches off the current established in the circuit inductance. Two disadvantageous effects are thereby incurred.Firstly, turning the series element on when the diode is in conduction results in a current surge through the series element which, in the case of a transistor for example, is limited only by its base drive and source inductance. Provision must therefore be made to dissipate this current surge. A second effect is that the sharp reduction to zero of the freewheel diode current results in the production of high voltages which, inter alia, act as a major source of radio frequency interference.

It is a primary object of the present invention to provide a means of reducing the aforegoing current surge. A secondary objective is to reduce the level of the reverse voltages produced at the freewheel diode and hence the level of R.F.I.

generated.

In accordance with the present invention, current surges in the series element are reduced by the incorporation of a saturating transformer which has two counterwound windings disposed on a high permeability core in series with the load, with the number of turns in the load winding (N,) exceeding that of the source winding (Ns), and with one side of the freewheel diode being connected to the junction of the two windings.

The arrangement is such that when the series element is non-conductive the load current flowing in the load winding (N,) maintains the core in saturation but when the series element becomes conductive once again and reverse current flows in the freewheel diode the transformer is taken into a high permeability region to limit the level of the latter current.

High voltages across the freewheel diode during collapse of the reverse current to zero are limited by the incorporation across the source winding (Ns) of a series circuit comprising a further diode and a resistor.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a simplified circuit diagram of the output end of a typical conventional chopper circuit; Fig. 2 shows the output waveform of the circuit of Fig. 1; Fig. 3 is a basic circuit diagram of the output end of a chopper circuit embodying the present invention; and Fig. 4 is a magnetisation curve used to explain the operation of the circuit of Fig. 3.

In the chopper circuit of Fig. 1, the average voltage applied to an inductive load L from a fixed d.c. supply is variable by controlling the on periods of a series element T1, which in this case is a transistor but which could equally well have been any other switching device, such as a thyristor, MOSFET, GTO and the like. Although fixed in this example, the d.c. supply could be variable. The switching characteristic of the transistor could also be of several forms, in that it could have a constant switching frequency with a variable mark-space ratio or it could be of variable frequency with a fixed pulse width, or a combination of the two last mentioned. Fig. 2 shows the resulting voltage waveform across the load L.

Such an arrangement might typically be used in deriving a variable voltage for driving a d.c.

motor or for deriving a variable voltage input to an inverter which drives an a.c. motor.

A freewheel diode D1 is disposed in parallel with the load L to provide a return path for the current which continues to flow in the inductive load L when the transistor T, is in its off condition.

Thus, when the transistor is off, the load current IL, which was previously flowing through the transistor to the load, then flows through the freewheel diode D1 and back to the load. As explained above, this arrangement has the problem associated with it that the diode D1 can still be in its conductive state when the transistor T, is switched on again. The forward current in the diode rapidly reduces to zero but its reverse blocking capability is not recovered immediately so that it conducts reverse current for a short time. This results in a current surge in the transistor T1 for which a means of current dissipation must be provided.

A number of known methods have been used to mitigate this problem. For example, the rate of change of current can be limited by the incorporation of an inductor in series with the collector of the transistor. However, such an inductor has to be by-passed by a resistor and a further diode in order to prevent "current-staircasing" and the generation of spike voltages. This method has the practical drawbacks that (a) substantial losses occur in the resistor; (b) spike voltages appear across the switching device; and (c) the mark space ratio cannot be raised to 100% since sufficient time must be allowed between pulses to dissipate the stored inductive energy in the inductance. It will be noted that considerations (b) and (c) above indicate diametrically opposite values for the bypass resistor and at best a compromise solution can be arrived at.

Fig. 3 shows an embodiment of a chopper circuit incorporating the present invention. The current includes a saturating transformer TF whose two windings, which are counterwound are connected in series with the load as shown in Fig. 3 with the anode of the diode D1 connected to the junction of the windings. The number of turns of the winding connected to the load (referred to as the load winding N,) is arranged to exceed that of the winding connected to the source (referred to as the source winding Ns).

When the transistor T1 is in conduction, load current 1, flows via the load, and the windings N, and Ns in series. In this situation, the core excitation for the transformer can be expressed thus: H=NLIL-NSlS (1) The core is chosen to be of high permeability material such that even a small load current will saturate the core in one direction.

On turning off the transistorT1, load current 1, will continue to flow via the free wheel diode D1, maintaining the core in saturation. Thus, there is no collapse of core flux and, if anything, the core is driven even further into saturation. There will, however, be some stored energy to be dissipated due to the self inductance of Ns but considered as an air cored inductor. This stored energy is dissipated in the series combination of a resistor R1 and diode D2 disposed in parallel with winding Ns.

When the transistor T, switches on again, the load current 1, will continue to flow via NL and Ns.

However, a reverse diode current 1R will flow via Ns for the recovery period of the diode D,. The core excitation under'these conditions is thus: H=NLlLNS(lL+lR) (2) The excitation goes negative when:

A negative excitation takes the core material out of saturation and into the high permeability region, as indicated at point P in Fig. 4. The impedance of the transformer to changes in 1R now becomes very high and so 1R will only increase slightly whilst the transformer is operating in this region. 1R is limited to the value calculated from equation (3) above, although a small ampere turns component must be added.

This takes the core into the active region.

The current 1R will flow into the diode D, until the stored charge has been swept out, at which point D, will no longer conduct reverse current.

However, this current 1R is established in transformer winding Ns and must be supplied with a path or high voltage will result. The voltage on the anode of diode D, now swings down and turns on the bypass path D2R1, the peak voltage cross R, being IRR,. The voltage across D, is thus limited.

By appropriate design of the transformer, the current 1R can be limited to say, 20% of the load current 1, plus the reset current which, for the particular core used in this embodiment, is about 2A. The value of R, should be low enough to limit the voltage across freewheel diode D, to below its breakdown voltage.

The diode current 1R will now die away and the transformer core will again be pushed into the saturation region.

Claims (Filed on 29 June 1983) 1. A chopper circuit comprising a switchable element disposed in series with an inductive load and including a saturating transformer which has two counterwound windings disposed on a high permeability core and connected in series with the load, with the number of turns in the load winding (N,) exceeding that of the source winding (Ns), and with one side of a freewheel diode being connected to the junction of the two windings, the arrangement being such that when the series element is non-conductive the load current flowing in the load winding (N,) maintains the core in saturation but when the series element becomes conductive once again and reverse current flows in the freewheel diode the transformer is taken into a high permeability region to limit the level of the latter current.

2. A chopper circuit as claimed in claim 1, further including the series combination of a diode and a resistor connected in parallel with the source winding (Ns) to provide a path for the reverse current (IR) which flows through the freewheel diode each time the series element is switched on.

3. A chopper circuit substantially as hereinbefore described with reference to and as illustrated in Figs. 3 and 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. to mitigate this problem. For example, the rate of change of current can be limited by the incorporation of an inductor in series with the collector of the transistor. However, such an inductor has to be by-passed by a resistor and a further diode in order to prevent "current-staircasing" and the generation of spike voltages. This method has the practical drawbacks that (a) substantial losses occur in the resistor; (b) spike voltages appear across the switching device; and (c) the mark space ratio cannot be raised to 100% since sufficient time must be allowed between pulses to dissipate the stored inductive energy in the inductance.It will be noted that considerations (b) and (c) above indicate diametrically opposite values for the bypass resistor and at best a compromise solution can be arrived at. Fig. 3 shows an embodiment of a chopper circuit incorporating the present invention. The current includes a saturating transformer TF whose two windings, which are counterwound are connected in series with the load as shown in Fig. 3 with the anode of the diode D1 connected to the junction of the windings. The number of turns of the winding connected to the load (referred to as the load winding N,) is arranged to exceed that of the winding connected to the source (referred to as the source winding Ns). When the transistor T1 is in conduction, load current 1, flows via the load, and the windings N, and Ns in series. In this situation, the core excitation for the transformer can be expressed thus: H=NLIL-NSlS (1) The core is chosen to be of high permeability material such that even a small load current will saturate the core in one direction. On turning off the transistorT1, load current 1, will continue to flow via the free wheel diode D1, maintaining the core in saturation. Thus, there is no collapse of core flux and, if anything, the core is driven even further into saturation. There will, however, be some stored energy to be dissipated due to the self inductance of Ns but considered as an air cored inductor. This stored energy is dissipated in the series combination of a resistor R1 and diode D2 disposed in parallel with winding Ns. When the transistor T, switches on again, the load current 1, will continue to flow via NL and Ns. However, a reverse diode current 1R will flow via Ns for the recovery period of the diode D,. The core excitation under'these conditions is thus: H=NLlLNS(lL+lR) (2) The excitation goes negative when: A negative excitation takes the core material out of saturation and into the high permeability region, as indicated at point P in Fig. 4. The impedance of the transformer to changes in 1R now becomes very high and so 1R will only increase slightly whilst the transformer is operating in this region. 1R is limited to the value calculated from equation (3) above, although a small ampere turns component must be added. This takes the core into the active region. The current 1R will flow into the diode D, until the stored charge has been swept out, at which point D, will no longer conduct reverse current. However, this current 1R is established in transformer winding Ns and must be supplied with a path or high voltage will result. The voltage on the anode of diode D, now swings down and turns on the bypass path D2R1, the peak voltage cross R, being IRR,. The voltage across D, is thus limited. By appropriate design of the transformer, the current 1R can be limited to say, 20% of the load current 1, plus the reset current which, for the particular core used in this embodiment, is about 2A. The value of R, should be low enough to limit the voltage across freewheel diode D, to below its breakdown voltage. The diode current 1R will now die away and the transformer core will again be pushed into the saturation region. Claims (Filed on 29 June 1983)

1. A chopper circuit comprising a switchable element disposed in series with an inductive load and including a saturating transformer which has two counterwound windings disposed on a high permeability core and connected in series with the load, with the number of turns in the load winding (N,) exceeding that of the source winding (Ns), and with one side of a freewheel diode being connected to the junction of the two windings, the arrangement being such that when the series element is non-conductive the load current flowing in the load winding (N,) maintains the core in saturation but when the series element becomes conductive once again and reverse current flows in the freewheel diode the transformer is taken into a high permeability region to limit the level of the latter current.

2. A chopper circuit as claimed in claim 1, further including the series combination of a diode and a resistor connected in parallel with the source winding (Ns) to provide a path for the reverse current (IR) which flows through the freewheel diode each time the series element is switched on.

3. A chopper circuit substantially as hereinbefore described with reference to and as illustrated in Figs. 3 and 4 of the accompanying drawings.