Help analyzing non-isolated ACDC circuit

Knight97538

Help analyzing non-isolated ACDC circuit [Copy link]

 

The following is a non-isolated ACDC circuit that my senior gave me. The ACDC input voltage is AC 180-260V and the output is 5V0.75A.

There are a few places I don't understand, can any teacher help me analyze it?

Q1 and Q3 in the box are dual-tube outputs, so what do C3 and D2 mean?

What is the function of D3 in the circle?

Knight97538

luck_gfb

C3 provides a voltage for the driver of the upper tube, and D2 charges C3 at the input end when Q4 is turned on. D3 is a freewheeling diode in the BUCK circuit.

luck_gfb

C3 provides a voltage for the driver of the upper tube, and D2 charges C3 at the input end when Q4 is turned on. D3 is a freewheeling diode in the BUCK circuit.

maychang

"What do C3 and D2 mean?"

As mentioned on the third floor, D3 is the freewheeling diode in the Buck circuit.

As for C3, it is indeed very dangerous. Can this schematic work without smoke or explosion?

The author should consider: How much can the two ends of C3 be charged to? Does it exceed the maximum value allowed by the gate of Q2?

Knight97538

luck_gfb posted on 2022-6-14 11:08 C3 provides a voltage for the driver of the upper tube, D2 allows Q4 to be turned on, and the input end charges C3. D3 is the freewheeling diode in the BUCK circuit.

Thank you teacher for your answer

Which components does D3 form the buck circuit?

Aren't freewheeling diodes usually Schottky?

Knight97538

luck_gfb posted on 2022-6-14 11:08 C3 provides a voltage for the driver of the upper tube, D2 allows Q4 to be turned on, and the input end charges C3. D3 is the freewheeling diode in the BUCK circuit.

This es1g is a fast recovery diode

In addition, why does Q2 above need to use freewheeling?

If I remove this diode, will there be no output?

Knight97538

maychang posted on 2022-6-14 11:40 "What do C3 and D2 mean?" As mentioned on the third floor, D3 is the freewheeling diode in the Buck circuit. As for C3, it is indeed very dangerous. This...

This es1g is a fast recovery diode

In addition, why does Q2 above need to use freewheeling?

If I remove this diode, will there be no output?

What is the maximum charge that the teacher said C3 can reach? What does this C3 maximum charge mean?

Knight97538

maychang posted on 2022-6-14 11:40 "What do C3 and D2 mean?" As mentioned on the third floor, D3 is the freewheeling diode in the Buck circuit. As for C3, it is indeed very dangerous. This...

I would also like to ask the teacher, for this kind of non-isolated ACDC, which does not have the cost of a transformer, how do we deal with safety?

Now the question is, when the reverse voltage of D3 diode is too large, will it be broken down and short-circuited, resulting in output short-circuit?

芯片老兵

Power supply is the area of concern at the moment, let's see if any experts can give me some advice.

superstar_gu

The key to this circuit is to understand the process of P MOSFET turning off.
Logically, when SW is low, P MOSFET is turned on; when SW is high, P MOSFET is turned off.
However, when the gate of P MOSFET is switched on and off, its parasitic capacitance needs to be quickly charged. This circuit converts AC110~230V to DC5V, and the switching rate is very low.
C3 and D2 act as temporary power supplies for transistors Q1 and Q3 locally, which are used to pull up or lower the gate voltage of P MOSFET (seems to be 12V).

superstar_gu

D3 is a fast recovery diode, which is used to block the input voltage from exceeding the rated setting value.

maychang

Knight97538 posted on 2022-6-14 13:21 This es1g is a fast recovery diode. In addition, why does Q2 above use freewheeling? If this diode is removed, will there be no output...

In addition, why does Q2 above need to use freewheeling?

If I remove this diode, will there be no output?

Because there is an inductor L1 in the circuit. The freewheeling is to the inductor L1, not to Q2.

If this diode is removed, when the current in Q2 is turned off, the induced electromotive force in the inductor L1 is quite high (theoretically infinite), and Q2 may be broken down. Even if Q2 withstands a very high voltage and is not broken down, the output voltage will be very small.

maychang

Knight97538 posted on 2022-6-14 13:26 In addition, I would like to ask the teacher, this kind of non-isolated ACDC, without the transformer cost, how to deal with the safety? Now the question is, will the D3 diode be used as...

"Also, I would like to ask the teacher, how to deal with the safety of this non-isolated ACDC without transformer cost?"

There is no security at all.

If an isolated flyback circuit is used, a low voltage of about 5V can also be output, but the more expensive P-channel tube is not used, but the cheaper N-channel tube is used. This reduces the cost. As for the transformer, it is only slightly larger than L1 in this circuit.

Isolated flyback circuit, a typical product is a mobile phone charger.

Therefore, this circuit can be said to be worthless.

Knight97538

superstar_gu published on 2022-6-14 14:22 The key to this circuit is to understand the process of P MOSFET turning off. Logically, when SW is low, P MOSFET is turned on; when SW is high, P MOSFET is cut off...

D2 mainly acts as a temporary power supply for Q1 and Q3. How do you understand this?

Knight97538

maychang posted on 2022-6-14 14:42 "In addition, I would like to ask the teacher, this kind of non-isolated ACDC, without the transformer cost, how to deal with the safety? 】 There is no safety at all. If...

It is unsafe without a transformer.

Can it be understood that all non-isolated ACDCs are unsafe?

maychang

Knight97538 posted on 2022-6-14 17:39 It is unsafe without a transformer. Can it be understood that all non-isolated ACDCs are unsafe?

"Can it be understood that all non-isolated ACDCs are unsafe?"

This cannot be understood.

Non-isolated ACDC converters are dangerous only when the AC side is AC mains (110V, 220V or 380V). If the AC side is isolated from the AC mains, there is no danger.

maychang

maychang posted on 2022-6-14 14:42 "In addition, I would like to ask the teacher, this kind of non-isolated ACDC, without the transformer cost, how to deal with the safety? 】 There is no safety at all. If...

The circuit in the first post is a typical Buck circuit. After the AC mains is rectified, the voltage is about 220~300V (the withstand voltage of C2C6 in the figure is 400V), and the output is 5V. Then the duty cycle can only be about 5/300=0.016. It is hard to imagine that PWM can achieve such a small duty cycle. Therefore, this circuit cannot stably output 5V, especially when the output is unloaded. So I said on the 14th floor: "This circuit is worthless."

luck_gfb

Knight97538 posted on 2022-6-14 13:19 Thank you for your answer. Which components does D3 form the buck circuit with? Shouldn't the freewheeling diode be a Schottky diode?

D3 should be a Schottky diode.

不亦心

maychang published on 2022-6-14 18:10 The first post circuit is a typical Buck circuit. The voltage of the AC mains after rectification is about 220~300V (the withstand voltage of C2C6 in the figure is 400V), and the output is 5V. Then the duty cycle...

Currently, there is a simpler high-voltage buck, which is generally used as an auxiliary power supply or in IOT application scenarios