Analysis and Implementation of a One-Ground-Diode Three-Switch Active-Clamp Forward Converter

A one-ground-diode three-switch active-clamp forward converter is proposed in this paper. The proposed converter exhibits the features of wide-input voltage-range application and high efficiency. All the switches of the proposed converter can achieve zero-voltage switching (ZVS) under turn-on transition. The operating principle of the proposed converter is presented in detail herein. A 400 V input voltage, 48 V output voltage, 400 W output power prototype of converter is implemented. Based on simulations and experimental results, the theoretical analysis is thus verified. Moreover, it reveals from experimental results that the maximum power efficiency of the proposed converter is 90.1 % at output power of 400 W.


Introduction
For the low-to-medium power applications, a forward converter is widely and often adopted because of its simplicity and high reliability.In the forward converter, the transformer is used to achieve galvanic isolation and high input to output step up or down in voltage.The flux of the magnetizing inductor should thereby be reset during a switching period in a steady-state operation to avoid transformer core saturation (1)(2)(3)(4)(5)(6)(7)(8)(9)(10).As a result, the tertiary reset winding (1-2), hybrid-bridge switching circuit (3)(4), RCD reset circuit (5)(6) and active-clamp reset circuit (7)(8) were proposed and applied to the forward converter.However, both the tertiary-winding forward converter and hybrid-bridge forward converter were limited to below a maximum duty cycle, which is 50 % sometimes, to insure proper reset of the transformer.RCD type forward converters were presented to enable stretching the maximum duty cycles above 50 %, particularly useful in wide range input supply designs.Nevertheless, the power consumption of the resistor in the RCD reset circuit degrades the overall efficiency of the converter.Moreover, the penalty for the high voltage stress on the semiconductors will also be paid with this adaptation.
In order to obtain the higher power density of a dc-dc power converter, the converter may operate at a higher switching frequency.However, the switching losses increase and the efficiency decreases with increasing frequency.To overcome this drawback, the soft switching techniques are taken into consideration (7)(8)(9)(10).As a result, a conventional active-clamp forward converter as shown in Fig. 1 is thus presented to obtain zero-voltage switching (ZVS) operation at turn-on transition for all the switches.
Unfortunately, an active-clamp forward converter still exhibits the drawback of the high voltage stress on the main switch.In order to share the high voltage stress of in V + Cc V , two main switches are thus applied.In practice, the voltage stress cannot be half shared because of the two unmatched main switches.It makes one of the two main switches may suffer from the full voltage stress of in V + Cc V .Therefore, a one ground diode is utilized to derive the proposed converter herein as depicted in Fig 2 .It reveals that the voltage stress of switch 1 S is clamped to Cc V , and the voltage stress of switch 2 S is clamped to in V .Notably, as the duty cycle is operated at 0.5, the voltage stresses across the main switches 1 S and 2 S are both equal to the input voltage.

Operating Principle
The proposed one-ground-diode three-switch active-clamp forward converter presented in Fig. 2  S .Some assumptions are made as follows before describing the operating principle of the proposed converter.
1) All switches and diodes of the proposed converter are ideal.The switching time of the switches and the reverse recovery time of the diodes can be neglected.
2) The time interval of the resonance and the charge and discharge time of capacitors Based on the switching of the switches and diodes, the proposed converter operating over one switching period S T can be divided into eight linear stages described as follows.The equivalent circuit of each stage is presented in Fig. 3 When the voltage In this stage, the transformer primary voltage is also clamped at zero, the currents of the rectifier diodes D s1 、D s2 are prepared to commutate.The stage ends when the main switch 1 S is turned off.v is discharge to zero simultaneously.In the meanwhile, the diode 3 D starts to conduct and the resonance is stopped.In this stage, we have To ensure the ZVS operation for the switch 3 S , the inductor current r L i is still negative at the end of this stage.Therefore, it obtains After the diode D s1 becomes reverse-biased, the transformer primary voltage is not clamped at zero anymore.Therefore, the inductor currents Lr i and Lm i are started to linearly decrease with the same slope of ) /( In this stage, the circuit operation is the same as the turn-off state of a conventional two-switch forward converter.) ( .Therefore, one gets   v decrease to zero.The diode 2 D is thus conducted, and this stage is finished.Similarly, the main switch 2 S can also be turned on under ZVS operation at the next stage.In this stage, we have In this stage, the inductor currents Lr i is started to linearly increase with the slope of .In this stage, the circuit operation is the same as the turn-on state of a conventional two-switch forward converter.
The next switching period starts when 2 S is turned on again.According to the aforementioned operating principle of the proposed converter, the key waveforms over one switching period S T are schematically depicted in Fig. 4.

Simulations and Experimental Results
For 400 V input and 48 V 400 W output, a prototype of the proposed one-ground-diode three-switch active-clamp forward converter, operating at 100 kHz, is implemented.Based on the simulations and experimental results, the soft-switching performance of all the switches and the high power efficiency of the proposed converter can thus be validated.The power specifications and component parameters are listed in Table 1.

Conclusions
A one-ground-diode three-switch active-clamp forward converter is proposed to achieve ZVS operation under turn-on transition for all switches.It can be used for wide-range input voltage and high efficiency applications.
The voltage stress across the two main switches can be approximately to the input voltage in V as the operated duty cycle 5 .0  D . The simulations and experimental results of a prototype with 400V input and 48 V/400 W output verify the theoretical analysis and performance of the proposed converter.The measured maximum power efficiency of the proposed converter is 90 % at output power of 400 W.

contains two main switches 1 S , 2 S
, one auxiliary switch 3 S in active-clamp circuit and one ground diode c D .The proposed converter exhibits the features of wide-input voltage-range application due to the unlimited duty cycle, high efficiency owing to the ZVS operation achieved for all switches and low voltage stress across the main switches 1 S , 2

1 :
on→off、 S 2 :off、 S 3 :off、 D c :on、 D 1 :off、 D 2 :off、D 3 :off、D S1 :on、D S2 :on ) As the diodes D s1 and D s2 both conducts, the transformer primary and secondary voltages are thus clamped at zero in this stage.The current is commutated form D s1 to D s2 , and the resonance between 1 :off、S 2 :off、S 3 :off、D c :off、D 1 :off、 D 2 :off、D 3 :on、D S1 :on、D S2 :on ) As the diode 3 D is forward-biased, the voltage 3 S C v is clamped at zero.The switch 3 S can be thereby turned on under ZVS operation.Because the voltage across to the inductor r L is Cc V  , the inductor current thus decreases linearly.During this stage, it yields

1 :
off、 S 2 :off、 S 3 :off→on、 D c :off、 D 1 :off、 D 2 :off、D 3 :on、D S1 :on、D :on) In this stage, the circuit operation is the same as that in stage 4. The current Lr i continues to fall linearly until max 5 ) ( Lm Lr i t i  at time 5 t .At this moment, the current commutation between D s1 and D s2 is also finished.Stage 6 [ 6 5 , t t ] (S 1 :off、S 2 :off、S 3 :on、D c :off、D 1 :off、 D 2 :off、D 3 :on、D S1 :on、D S2 : off)

1 :
off、S 2 :off、S 3 :off、D c :off、D 1 :off、 D 2 :off、D 3 :off、D S1 : on、D S2 :on) As the diode 3 D conducts, transformer primary voltage is clamped at zero.It makes the current commutation between D s1 and D s2 occur.The capacitor voltages falls to zero, the diode 1 D becomes forward-biased and this stage ends.

2 S 1 : 1 :
are turned on under ZVS condition at time 10 t .Notably, it reveals that the ZVS operation for switches 1 S and 2 S can be achieved if the following inequality equation should be satisfied.on、S 2 :on、S 3 :off、D c :off、D 1 :off、 D 2 :off、D 3 :off、D S1 :on、D S2 :on) In this stage, the circuit operation is the same as that in stage 10.The current Lr i continues to rise linearly until commutation between D s1 and D s2 is thus finished.on 、S 2 : on、S 3 :off、D c :off、 D 1 :off、D 2 :off、D 3 :off、D S1 :off、D S2 :on)After the diode D s1 becomes reverse-biased, the inductor currents Lr i and Lm i are started to linearly increase with the same slope of )

Fig. 4 .
Fig.4.Key waveforms over one switching period s T .

3. 2 3 . 3
Figs. 6-8 show the simulations and experimental results of drain-to-source voltages and gating signals of all the switches.It reveals that the switches are turned on after their drain-to-source voltages drop to zero.As a result, all the switches of the proposed converter are achieved ZVS operation at turn-on transition.Moreover, we also can find that the maximum voltages across switches 1 S and 2 S are nearly input voltage V 400  in V .3.3 Power efficiency measurementFig.9showsthe measured efficiency versus various output power.The efficiency at different load conditions is also listed in the Table2.The highest efficiency of the proposed converter is about 90.1 %.The efficiency of the proposed converter can be further improved by using synchronous rectification.

Fig. 6 .
Fig. 6.Waveforms of gating signal and drain-to-source voltage of switch 1 S at full load.

Fig. 7 .
Fig. 7. Waveforms of gating signal and drain-to-source voltage of switch 2 S at full load.

Fig. 8 .
Fig. 8. Waveforms of gating signal and drain-to-source voltage of switch 3 S at full load.

Table 1 .
Power specifications and component parameters.