AT525013A1 - Combined flyback-forward converters with square buck preamp - Google Patents

Abstract

The invention relates to two variants of a combined flyback-forward converter with a quadratic step-up/step-down preliminary stage, consisting of a positive (1) and a negative connection (2) for supplying the input voltage (U), a positive (3) and a negative connection (4 ) for connection to the load (R), an active switch (S1), a first diode (D), a second diode (D), a third diode (D3) and optionally a fourth diode (Da), a first (Li ) and a second coil (L), a magnetically coupled coil (T) with a first (Nu) and a second winding (NM), a first (Ci), a second (C) and optionally a third capacitor (C3). The energy transfer to the secondary side takes place both in the conducting and in the blocking state of the clock period of the active switch.

Description

The invention relates to a combined flyback-forward converter with a square step-up pre-stage, consisting of a positive (1) and a negative connection {2} for supplying the input voltage (U4), a positive (3) and a negative connection (4) for connection to the load {R}, an active switch (Sı}, a first diode (Di), a second diode {D2), a third. diode (Da) and optionally a fourth diode (Da), a first {L;) and a second coil {L2}, a magnetically coupled coil {T} with a first (N14) and a second winding (N2), a first (C,), a second (C,) and optionally a third capacitor (C3), the cathode of the second diode (D,) and the first terminal of the first coil (L.) being connected to the positive terminal {1} , to the anode. the second diode (D:} the cathode of the first diode (Da) and the second connection of the first capacitor (C,) are connected, to the first connection of the first capacitor (C.) the second connection of the first coil (Li) and the positive terminal of the active switch (Sı) are connected, and the negative terminal of the active switch (S;) and

the negative terminal (2) are connected,

Flyback and forward converters make it possible to achieve large voltage conversions through the turns ratio of the windings. At the same time, they also enable potential isolation between input and output. In barrier mode, energy is drawn from the source when the active switch is on; the energy is stored in the magnetic circuit of the coupled coils and when the active switch blocks, this energy is released to the output circuit via the second winding. Due to the required intermediate storage in the magnetic field, the flyback converter concept is only suitable for outputs of up to around 150 W. Larger powers are then transmitted with the flow converter. Here the two coupled coils work like a transformer. If the active switch is switched on, energy is also immediately transferred to the secondary circuit according to the transformer principle. As with a transformer, a magnetizing current also flows on the primary side in addition to the current required for the current on the secondary side. After switching off the active switch, this is reduced with the help of a third magnetically coupled winding. With this one

However, the proposed concept does not require any additional winding for demagnetization,

To extend the flexibility and the voltage transformation ratio is a quadratic

Added stage with up-down behavior.

The two proposed converter structures will now be described with reference to figures. fig, 1

Variant 1 and FIG. 2 represent variant 2. FIGS. 3 to 5 and 6 to 8 represent

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undesired switching on of the first diode {D1}.

Fig. 1 shows variant 1. This circuit requires four diodes and two capacitors, Fig. Z represents variant 2. Here you only need three diodes but also three capacitors. Otherwise, both circuits have the same number of components: two sinks {L;. L2}, a coupled coil {T} with two windings (N11, Nu) and an active switch {S;). The active switch is an example

drawn as MOSFET,

The function of the circuits will now be discussed separately for the two variants for easier understanding. Steady-state operation and ideal components are assumed. The capacitors are suitably large, so the voltage across them during a clock period

can be assumed to be practically constant.

In order to describe the function of the circuit according to FIG. 1 variant 1, the individual modes of the circuit are now considered. Fig, 3 represents mode M1, the active switch {S1) is on, so that the input voltage on the first coil {L;) and the current through it decreases. to, When the active switch (5;) is switched on, the first diode (Di) also switches on. As a result, the voltage of the first capacitor (C1) is applied to the primary winding (N41) and generates a voltage on the secondary winding (N12) which transforms according to the turns ratio

is, according to

Thy AD

This switches the third. Diode (D2} on and the secondary voltage is now on the second coil {L:) and the current through it increases. This secondary current now causes a corresponding current on the primary side {changed by the factor Nu/Nı: according to the turns ratio}, which also flows through the transistor. The energy is taken from the first capacitor (C;) {the voltage across C, increases therefore practically something ab}. The load will be in this Madus

powered by capacitor C.

If the active switch (5;} is now switched off by the control device, mode MZ {FIG. 4) begins. The current through the first coil (L.) now commutes in the second diode (D,). This means that there is a negative voltage (the voltage at Ci} on the coil and the current drops. At the same time, the first diode (D1) switches off and current can no longer flow in the primary winding. However, the magnetizing current can flow through the secondary winding (N12 } dissipate. For this purpose, the fourth diode (Da) switches on. The magnetizing current now builds up via the circuit of the secondary winding (N;2), second capacitor (C2) and fourth diode (Da).

T57/fh/20210501

on the second. capacitor {C,) and is supplied from there.

If the currents through the secondary winding (N1z) and through the second coil (L1) reach zero, the third (Da) and the fourth diode (Da) switch off. The mode M3 according to FIG. 5 now begins. The first coil (L:) continues to build up its magnetization and the second capacitor (C,) supplies the

Load.

If the current in the first coil {Li} also reaches the value zero, the second diode (D») also switches.

off, This is MA mode. The load continues to be supplied by the second capacitor {C,),

It should also be noted that there are other possibilities, e.g. the current in the first coil (L,} can also start to gap before that in the second coil {12}, or the active switch {S:) even before the coupled coils (T)} or the second coil {L,}) have demagnetized again

is switched on,

However, the sequence of modes M1, M2, M3 is the easiest to control and the risk of saturation is avoided, but that doesn't mean that the circuit can't be done differently

can operate.

The DC voltage transformation ratio in continuous operation can be calculated according to

Me lb Z

EL Na | d U rare

determine, One recognizes the function of a quadratic buck converter which according to the

turns ratio can be adjusted,

In discontinuous operation, the formula is of course not that simple. The switching frequency f, the load {R} and the size of the primary winding (Lux) and the value of the second coil {L;) are also included. The value of the first coil (L:} is not included here, because the first coil (L,} is also used in

this case better in continuous. Operate mode, It turns out

M =

DD A Kai U 1

With. the factor Koem, in which the parameters of the converter are summarized, This results

to

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Nız Bf mm \”al

The circuit variant 2 according to FIG. 2 is now also based on the modes

Kpcar S

described. Mode M1 (Fig. 6) starts by turning on the active switch {Sy}. As a result, the input voltage is applied to the first coil (L.} and the current {in the coil increases (with a constant input voltage, the increase is near). At the same time, the diode (D4)} also turns on and at the primary winding (Nas) the coupled coils {T} is then the voltage of the first capacitor {C:). This creates the voltage again on the secondary winding {N} of the coupled coils {T} according to the turns ratio

7 =} Nız AA TSCE Rr N

The third diode {D:) blocks and the voltage

FE Ui SU De

Is now present in the second coil {L,} and leads to an increase in current (if the capacitors are dimensioned sufficiently large, so their voltage hardly changes during the period, this increase is also linear), the current in the primary winding now increases by the factor

Aa/Naz transforms on.

Madus M2Z starts turning off the active switch (Sı}. This turns on the second diode (D2) and takes over the current of the first coil (Li}. The voltage on the first coil {L:} is now the negative voltage of the first capacitor (Ci) and the current in the first coil {L,} decreases. At the same time, the first diode (Di) switches off and thus interrupts the current flow in the primary winding (Nur), This also stops the energy transfer to the forward converter principle of the coupled coils (T} is now transmitted according to the barrier principle. The third diode (D:) switches on, the magnetizing current builds up via the second capacitor (C,), the third diode (Da) and the secondary winding (Nız). At the same time, the current of the second coil (L,) runs through the third diode (Ds} and the third

Capacitor {C3} free.

It makes sense to select the same size for the inductance of the second coil {L,} and the inductance (Lex) of the secondary winding (N) so that both coils are demagnetized at the same time. In this mode both the second (C;) and the third capacitor (C3}

reloaded.

T5S7/fh/20210501

operate in such a way that the first coil (Li) remains in continuous operation.

The DC voltage transformation ratio in cantilever operation can be calculated according to

£ < 7 ar f \Ur Naf_d X

Man ns

EU Nall-d)

determine and shows the same result as varlante 1, you can see the function of a

square Hach step-down converter which can still be adjusted via the turns ratio.

Of course, the formula is not that simple in discontinuous operation. The switching frequency £, the load {R} and the size of the primary winding (Ly:) and the value of the second coil {L,) are also included. The value of {L,) is not included here, because in this case it is better to operate the first coil in the cantilever mode. The result is again the same

Voltage transformation ratio as in Variant 1.

It is known that high-frequency oscillations occur in converters that are operated in the discontinuous mode. the switches, conditionally. due to their parasitic capacitances. If you want to suppress these vibrations for reasons of interference suppression, then bid. There are two possibilities, on the one hand you can short-circuit the first winding (N41} of the coupled coils (T} during the M3 mode, or you can extend the second coil {L,} with a second winding (a thin wire is sufficient, because there flowing current has only a low effective value) and closes it

second winding short during M3.

In the first variant, for example, you switch to a MOSFET whose source is connected to the first terminal of the first winding (Nu) of the coupled coils (T}, another diode whose anode is connected to the second terminal of the first winding (Na) of the coupled coils {T}, This arrangement means that no potential-free control is required for this auxiliary transistor.The diode connected in series prevents the baddydiade of the

MOSFETS.

In the second variant, no potential-free control is required either. The two winding ends are each connected to a positive connection of an active switch.

These two auxiliary switches are connected together with their negative terminals, This is then

T57/fh/20210501A 6/16

Mass can be controlled.

It should also be noted that with certain duty cycles the first diode (D1} can become conductive outside of the M1 mode. To avoid this, another active switch (Fig. 9), which is identical to the main switch (S1), can be used. Is driven, in series to

switch on the first diode (D1}.

The task of realizing a combined flyback-forward converter with a quadratic step-up/step-down behavior is achieved according to the invention in that the second terminal of the first winding {N} of the coupled coil (T) is connected to the anode of the first diode (Di }) and the first terminal of the first winding (N4;) of the coupled coil {T} is connected to the negative terminal of the active switch (S;), to the second terminal of the second winding (Na2) of the coupled coil {T} the second terminal of the second coil {L,), the first terminal of the second capacitor {C,} and the positive output terminal {3} are connected, to the first terminal of the second winding (N12)} of the coupled coil {T} the anode of the third diode (Da } and the cathode of the fourth diode (Da) are connected, to the cathode of the third diode (D3} the first terminal of the second coil (L;) is connected and to the anode of the fourth diode {D}, the second terminal of the secondCapacitor (C,} and the negative output terminal (4} are connected, or that the second terminal of the first winding (Nu) of the coupled coil {T} and the first terminal of the first winding are connected to the anode of the first diode (Di). (Na) of the coupled coil {T} is connected to the negative terminal of the active switch (S;}, to the second terminal of the second winding {N42) of the coupled coil (T} the first terminal of the second capacitor (C;) is connected, the second terminal of the second capacitor (C:} is connected to.the second terminal of the second coil {L2} and the anode of the third diode (D3}, the first terminal of the second coil (L2) to the second terminal of second capacitor {C,} and the negative output terminal (4), and the first terminal of the second winding {Nx2) of the coupled inductor (T} to the cathode of the third diode (D3}, the first terminal of

third capacitor (C3) and the positive output terminal (3}, the active switch can also be protected by means of a snubber network,

In order to avoid the influence of the lead inductance, parallel to the

Input terminals (1, 2} switch a capacitor (or a combination of capacitors).

In order to avoid disturbing oscillations in the semiconductors, parallel to the first winding {N:1) of the coupled coils {T}, a further diode and a further diode can be connected in series

Switch active switch, the negative terminal of the other active switch to the

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coupled coils {T}.

These oscillations can also be avoided by having a second winding magnetically coupled to the winding of the second coil {L;}, the first connection of this second winding being connected to the series circuit of a further diode and a further active switch, the negative connection of the further active switch is connected to the second connection of this second winding and to the positive (3) or the negative

Output terminal (4} is switched.

An alternative variant is that there is a second winding magnetically coupled to the winding of the second coil {L;}, where the first connection of this second winding is connected to the series circuit of a further diode and a further active switch, where the negative connection of the further active switch is connected to the second terminal of this second winding and to the positive {3} or the negative output terminal (4)

is switched.

To prevent the first diode {D1} from conducting under certain conditions, one can. complement the circuit in that in the series connection of the first diode (Di) and the

first winding (Nı,) of the coupled coils (T)} an active switch (5,) is connected,

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Claims (1)

2. Combined flyback-forward converter with quadratic step-down pre-stage, consisting of a positive {1} and a negative terminal (2) for supplying the input voltage. (Uh}, a positive (3) and a negative terminal {4} for connection to the load {R}, an active switch (Si), a first diode (D4)}, a second diode (D,}, a third diode (D3} and optionally a fourth diode {Da}, a first (L;) and a second coil (L2), a magnetically coupled coil (T} with a first (Nu) and a second winding {N4), a first {C,), a second {C2} and optionally a third capacitor (C,}), where the positive terminal (1) is connected to the cathode of the second diode (D,}) and the first terminal of the first coil {L,} are connected, to the anode of the second diode {D.) the cathode of the first diode (Di) and the second terminal of the first capacitor (Ci) are connected, to the first terminal of the first capacitor (Ci) the second terminal of the first Coil (L:) and the positive terminal of the active switch (Sı) are connected, and the negative terminal of the active switch (Sı) and the negative terminal (2) ve Connections are characterized in that the second connection of the first winding {N.1) of the coupled coil (T} is connected to the anode of the first diode (Di) and the first connection of the first winding {N} of the coupled coil {T} connected to the negative terminal of the active switch (Sı), to the second terminal of the second winding {N} of the coupled coil {T}, the second terminal of the second coil {L2), the first terminal of the second capacitor (C>} and the positive output terminal (3) is connected:the anode of the third diode (D3} and the cathode of the fourth diode (Da} are connected to the first terminal of the second winding (N2) of the coupled coil {T}, to the cathode of the third diode {Da} the first terminal of the second coil {L} is connected and to the anode of the fourth diode (Da), the second terminal of the second capacitor (C2} and the negative output terminal (4} are connected, or that at the anode of the first diode {Di) the second A terminal of the first winding (N) of the coupled coil {T} and the first terminal of the first winding (Nu) of the coupled coil (T} is connected to the negative terminal of the active switch (Si), to the second terminal of the second Winding (N,2) of the coupled coil {T} the first terminal of the second capacitor (C,) is connected, the second terminal of the second. Koondensators (C2} to the second terminal of the second coil {L2}) and the anode of the third diode (D3} is connected, the first terminal of the second coil (L;}) to the second terminal of the second capacitor (C;) and the negative output terminal (4) is switched, and the first connection of the second winding (N12) of the coupled coil {T} to the cathode of TS7/fh/20210501 2. Converter according to claim 1, characterized in that the active switch is protected with a load-relieving network. 3, Converter according to claims 1 and 2, characterized in that a capacitor is connected in parallel to the input terminals (1, 2}. 4. Converter according to claims 1 to 3, characterized in that the series connection of a further diode and a further active switch is connected in parallel with the first winding (N44) of the coupled coils {T}, the negative connection of the further active switch being connected to the negative Connection of the input (2} and the first connection of the first winding (N) of the coupled coils {T} is connected, 5. Converter according to Claims 1 to 3, characterized in that a second winding is magnetically coupled to the winding of the second coil (L;), the first connection of this second winding being connected to. the series connection of a further diode and a further active switch is connected, the negative terminal of the further active switch being connected to the second terminal of this second winding and being connected to the positive {3} or the negative output terminal (4}. 6. Converter according to claims 1.bis 3 characterized in that with the winding: the second coil {L,} a second magnetically coupled winding is present, the terminals of this second. Winding are each connected to the positive terminals of further active switches whose negative terminals are switched together either to the positive (3) or the negative output terminal {4}. 7. Converter according to any one of claims 1 to 6, characterized in that in the series circuit of the first diode (D4) and the first winding (Nu) of the coupled coils {T} an active switch (S,) is switched. T57/fh/20210501