Communication switching power supply technology introduction (2)

100V, 250mA, surface mount, high speed switching diode

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Summary:

In the initial stage of the development of the switching power supply, the power switch tube is turned on or off in a state where the voltage or current on the device is not zero. That is, the device is forced to turn on when the voltage on the device does not reach zero voltage, forcing the device to turn off when the current flowing through the device does not reach zero current. Since the switching transistor is not an ideal device, there is an overlap between the current and the voltage during the turn-on and turn-off periods, which causes loss. This operating state is called "hard switching". The power switch that operates in the hard-switching state has a large loss, and as the switching frequency increases, the switching loss also increases. Therefore, the hard switching technology limits the operating frequency and efficiency of the switching regulator power supply.

The emergence of soft switching technology has effectively solved this problem. The so-called "soft switch" refers to Zero Voltage Switching (ZVS) or Zero Current Switching (ZCS), which is a resonant principle applied to change the voltage (or current) in a switching device according to a sinusoidal law. When the voltage (or current) is zero, the device turns on (or turns off), reducing the overlap of voltage and current. In this way, the theoretical switching loss can be zero. Applying soft switching technology, the operating frequency of the switching power supply can reach the order of MHZ. According to the control mode, the soft switch can be generally divided into three types: pulse width modulation (PWM), pulse frequency modulation (PFM), and pulse phase shifting (PS).

1 PWM converter

The PWM control mode means that the stable output is realized by adjusting the pulse width in the early stage of the constant operating frequency of the switching tube. This is the most widely used method for medium and small power switching power supplies.

1.1 Zero Current Switching PWM Converter

Figure 1 Buck type ZCS-PWM converter

Figure 1 shows a Buck-type zero-current switching converter with additional auxiliary switching control. The working process is slightly different from the previous process:

Linear phase (S1, S2 conduction): At the beginning, under the action of LR, S1 zero current is turned on. Subsequently, due to the action of Uin, the ILR rises linearly and reaches ILR=I0.

Forward resonance phase (S1, S2 on-off): When ILR = I0, since CR starts to generate voltage, VD naturally turns off at zero current. After that, LR and CR begin to resonate. After half a resonance period, ILR re-rescillates to I0, UCR rises to the maximum value, and ICR is zero, S2 is turned off, UCR and ILR will be maintained, and resonance cannot continue.

Hold phase (S1 turn-on, S2 turn-off): This state hold time is determined by the PWM circuit requirements. During the hold period, Uin normally supplies power to I0.

Reverse resonance phase (S1 turn-on, turn-off, S2 turn-on): When S1 needs to be turned off, it can be controlled to re-open S2. At this time, under the action of LR, the S2 current is 0. The resonance begins again, and when the ILR is back-resonant to zero, S1 can be turned off at zero current and zero voltage.

Recovery phase (S1 is off, S2 is on): After that, UCR decays to 0 under the action of I0.

Freewheeling phase (S1 off, S2 on-off): After the UCR decays to zero, the VD naturally turns on and begins to flow. Due to the short circuit of VD, S2 can be turned off in zero voltage and zero current mode before the next cycle.

It can be seen that S1 is turned on in the first four stages (linear, resonant, and hold), and is turned off during recovery and freewheeling. The role of S2 is mainly to block the resonance generation and hold phase. The effective control of S1 and S2 produces the effect of PWM and realizes its own soft switching by using resonance.

The switching tube and the diode of the circuit are all turned on and off under zero voltage or zero current condition, and the main switching voltage stress is low, but the current stress is large (resonance effect). The freewheeling diode has a large voltage stress and the resonant inductor is on the main path, so the load, input, etc. will affect the ZCS operating state.

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