What is the switching power supply with constant power output?

wxkang

What is the switching power supply with constant power output? [Copy link]

 

Does constant power output mean that a fixed power is always output? Will it change with load changes? If the input of a flyback switching power supply is 12V, I sample a current at the MOS tube and compare it with my set value. Is this sampled value consistent with my target value? Is this considered a constant power switching power supply?

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maychang

wxkang posted on 2020-12-7 10:45 Like it

“Does constant power output mean that a fixed power is always output? Will it change with load changes?”

It always outputs a fixed power. Within a certain range, the output power does not change with the load.

maychang

wxkang posted on 2020-12-7 10:45 Like it

"If the input of a flyback switching power supply is 12V, I sample a current at the MOS tube and compare it with my set value. Is this sampled value consistent with my target value? Is this considered a constant power switching power supply?"

Such a flyback switching power supply has approximately constant power output.

wxkang

maychang published on 2020-12-7 12:32 "If the input of a flyback switching power supply is 12V, I sample a current at the MOS tube and compare it with my set value. Is this sampled value and my target...

This will cause a problem, that is, when my power supply is at a constant power of 50W and the load is 40W, will it cause the output voltage of the power supply to increase and burn out the device?

maychang

wxkang posted on 2020-12-7 13:39 This will cause a problem, that is, when my power supply has a constant power of 50w and the load is 40w, will it cause the output side voltage of the power supply to increase and burn out the device?

"My power supply has a constant power of 50W and the load is 40W. Will this cause the output voltage of the power supply to increase and burn out the device?"

Burn out the device? You mean burn out the load?

wxkang

maychang published on 2020-12-7 14:38 "My power supply has a constant power of 50w and a load of 40w. Will this cause the output voltage of the power supply to increase and burn out the device?" Burn out the device...

Yes, the flyback power supply has a constant power output of 50W. If the load is 40W, the voltage at the output end of the power supply should become very high, thereby burning out the load.

maychang

wxkang posted on 2020-12-7 15:15 Yes, the flyback power supply has a constant power output of 50W. If the load is 40W, then the voltage at the output end of the power supply should become very high, thereby burning out the load.

For a constant power output power supply, its output power must be adjustable. You only need to lower the output power (for example, to 30W) to ensure that the load will not be damaged.

wxkang

maychang published on 2020-12-7 14:38 "My power supply has a constant power of 50w and a load of 40w. Will this cause the output voltage of the power supply to increase and burn out the device?" Burn out the device...

It is also possible that the MOS tube is burned. When the input power remains unchanged, the duty cycle remains basically unchanged. When the output voltage becomes higher, the reflected voltage will become higher, so the withstand voltage of the MOS must be high, otherwise the MOS will break down. Is this right?

wxkang

maychang posted on 2020-12-7 15:17 For a constant power output power supply, its output power must be adjustable. You only need to lower the output power (for example, to 30W) to ensure that the load will not be damaged.

As shown in the figure above, I use 12v and I_check to calculate the power and make a constant power output of 50w. When the load becomes 40w, it is detected that v_check increases. At this time, the PWM duty cycle is reduced until the output voltage is stable. If so, how can it feel like a constant voltage output?

maychang

wxkang posted on 2020-12-7 15:19 It is also possible to burn the MOS tube. When the input power remains unchanged, the duty cycle remains basically unchanged. When the output voltage becomes higher, the reflected voltage will become higher, so the MOS endurance is required...

When the flyback converter works intermittently, the output voltage becomes higher, and the voltage reflected to the primary of the transformer also becomes higher, which may damage the power switch tube. This must be taken into account in the design, and the duty cycle should be reduced to limit the output power (of course, the output voltage is also limited) when the output voltage is higher than a certain value.

maychang

wxkang posted on 2020-12-7 15:35 As shown in the figure above, I use 12v and I_check to calculate the power and make a constant power output of 50w. When the load becomes 40w, it is detected that v_check increases, at this time...

Without seeing the complete feedback circuit, it is impossible to say whether it is a constant voltage output.

The constant power output mentioned above is determined by the characteristics of the flyback converter in the discontinuous current working state: if the DC power supply voltage remains unchanged and the duty cycle remains unchanged, the output power remains unchanged when the load changes.

Now according to your diagram, there is feedback. If the output voltage is sampled from the V_chech point to achieve feedback, then the circuit has a constant voltage output.

wxkang

maychang posted on 2020-12-7 16:01 Without seeing the complete feedback circuit, it is impossible to say whether it is a constant voltage output. The constant power output mentioned above is the flyback converter in the intermittent current working state...

v_check is input to the microcontroller, which outputs a current reference value to the mos driver chip to generate the corresponding pwm

maychang

wxkang posted on 2020-12-7 16:09 v_check is input to the microcontroller, and the microcontroller outputs a current reference value to the mos driver chip to generate the corresponding pwm

If the microcontroller only samples the output voltage but not the current, then the flyback switching power supply controlled by this microcontroller can only have a regulated output.

If both the output voltage and the output current are sampled, and a multiplication is performed in the microcontroller, the product is compared with the reference value, and then the switching power supply is controlled, then the microcontroller-controlled flyback switching power supply has a constant power output.

wxkang

maychang published on 2020-12-7 17:38 If the microcontroller only samples the output voltage but not the current, then the flyback switching power supply controlled by this microcontroller can only be a regulated output. If...

The single-chip microcomputer collects the input voltage and the output voltage, and then outputs a PWM wave through RC, converting the PWM into a DC voltage signal, which is compared with the voltage value of the current sampled at the S pin of the MOS tube. So I guess that this power supply has a fixed output power of 50W, and the collected input voltage and power are calculated to get the set current. In this case, the collected output voltage is used for protection, and the power supply is turned off when the output voltage is too high. Then such a constant power supply only ensures that the set power is constant, and the actual output power deviation should be relatively large.

maychang

wxkang Published on 2020-12-8 09:33 The microcontroller collects the input voltage and the output voltage, and then the microcontroller outputs a PWM wave through RC, converting PWM into a DC voltage signal, which...

"The microcontroller collects the input voltage and the output voltage, and then outputs a PWM wave through RC, converting the PWM into a DC voltage signal. This DC signal is compared with the voltage value of the current sampled at the S pin of the MOS tube."

In this case, the circuit including the microcontroller is likely designed for constant power output. The microcontroller may multiply the collected output voltage and output current, then compare the product with a reference value, and output a PWM wave based on the difference.

maychang

wxkang Published on 2020-12-8 09:33 The microcontroller collects the input voltage and the output voltage, and then the microcontroller outputs a PWM wave through RC, converting PWM into a DC voltage signal, which...

"So I guess this power supply has a fixed output power of 50W, and the input voltage and power are collected to calculate the set current."

If this power supply is a constant power output, then the voltage peak across the current sampling resistor on the source of the MOS tube is directly related to the output power. This flyback switch power supply control chip is mostly a dual-loop control chip such as UC3842. As for how much the output power is, it depends on how much the internal reference of the microcontroller is set to. In fact, this internal reference is a value in the memory. Modifying this value in the memory will change the output power.

maychang

wxkang Published on 2020-12-8 09:33 The microcontroller collects the input voltage and the output voltage, and then the microcontroller outputs a PWM wave through RC, converting PWM into a DC voltage signal, which...

"Then such a constant power supply only ensures that the set power is constant, and the actual output power deviation should be relatively large."

If the microcontroller multiplies the collected output voltage and output current and then compares them with the reference value, the power error of the switching power supply output can be very small, 1% can be easily achieved, and higher values are also possible.

wxkang

maychang published on 2020-12-8 10:58 "Then such a constant power supply only ensures that the set power is constant, and the actual output power deviation should be relatively large." If the single chip...

The MCU of this power supply does not collect the output current, but only the output voltage and the supply voltage. The MOS source voltage is also collected directly to the MOS driver chip, not to the MCU.

maychang

wxkang Published on 2020-12-9 09:05 The microcontroller of this power supply does not collect output current, but only collects output voltage and supply voltage. The mos source voltage is also collected directly to the mos driver chip...

"The microcontroller of this power supply does not collect output current, but only collects output voltage and supply voltage"

If the output current is not collected, the possibility of constant power output of the flyback switching power supply cannot be said to be zero, but it is smaller than the possibility of constant power output by collecting the output current. If the output current is not collected, the constant power output can be achieved entirely by keeping the duty cycle unchanged and the output power unchanged.