Practical Tips | Break down the power circuit layer by layer, accurately to each component
Latest update time：2022-04-02
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This power supply explanation takes a 13.2W power supply as an example
Input: AC90~264V
Output: 3.3V/4A
Schematic
The transformer is the core of the entire power supply, so the calculation and verification of the transformer is very important.
Determine the material and size of the transformer:
According to the transformer calculation formula
Determine the primary filter capacitor:
The decision of the filter capacitor can determine the Vin(min) on the capacitor. The larger the filter capacitor, the higher the Vin(win), which can produce a larger wattage of power, but the relative price is also higher.
Determine the transformer wire diameter and number of wires:
Once the transformer is determined, the Bobbin of the transformer can be determined. Based on the slot width of the Bobbin, the wire diameter and number of wires of the transformer can be determined, and the current density of the wire diameter can also be calculated. The current density is generally based on 6A/mm2. For the design of the transformer, the current density can only be used as a reference value, and the temperature rise record should be the final standard.
Determine the Duty Cycle:
The duty cycle can be determined by the following formula. The duty cycle is generally designed based on 50%. If the duty cycle exceeds 50%, oscillation may occur.
Determine IP value:
Determine the number of turns of auxiliary power:
The number of turns and voltage of the auxiliary power supply can be determined based on the turns ratio of the transformer.
Determine the stress of MOSFET and secondary diode:
Based on the transformer's turn ratio, we can preliminarily calculate whether the transformer's stress meets the specifications of the selected parts. The calculation is based on an input voltage of 264V (380V on the capacitor).
other:
If the output voltage is below 5V and TL431 must be used instead of TL432, an extra set of windings must be considered for use with the photo coupler and TL431.
Substitute the obtained data into
In the formula, B(max) can be obtained in this way. If the B(max) value is too high or too low, the parameters must be readjusted.
Transformer calculation:
Output wattage 13.2W (3.3V/4A), Core = EI-28, Windable area (slot width) = 10mm, Margin Tape = 2.8mm (each side), Remaining windable area = 4.4mm.
Transformer material and size:
From the above assumptions, we know that the material is PC-40, the size = EI-28, Ae = 0.86cm2, the windable area (slot width) = 10mm, and because the Margin Tape uses 2.8mm, the remaining windable area is 4.4mm.
Assume that the filter capacitor uses 47uF/400V, and Vin(min) is temporarily set to 90V.
Determine the wire diameter and number of wires of the transformer:
Determine the duty cycle:
Determine IP value:
Determine the number of turns of auxiliary power:
Determine
the stress of MOSFET and secondary diode:
other:
Because the output is 3.3V and the Vref value of TL431 is 2.5V, if the voltage drop of about 1.2V on the photo coupler is added, the output voltage will not be able to drive the photo coupler and TL431, so an additional set of coils must be added to provide the voltage required for the feedback path.
Transformer wiring diagram:
Parts selection:
FS1:
The Iin value is calculated from the transformer. Based on this Iin value (0.42A), it can be known that the company's shared material 2A/250V is used. When designing, it is also necessary to consider whether the Iin at Pin (max) will exceed the rated value of the fuse.
TR1 (thermistor):
At the moment of power activation, due to the short circuit of C1 (primary filter capacitor), the Iin current is very large. Although the time is very short, it may also cause damage to the Power. Therefore, a thermistor must be installed before the filter capacitor to limit the Iin within the Spec (115V/30A, 230V/60A) at the moment of power on. However, because the thermistor also consumes power, the resistance value cannot be too large (otherwise it will affect the efficiency). Generally, SCK053 (3A/5Ω) is used. If a larger value of C1 capacitor is used, the resistance value of the thermistor must be increased (generally used on large-wattage Power).
●VDR1(Surge Absorber):
When lightning strikes, it may damage components and affect the normal operation of the power supply. Therefore, a surge absorber must be added to the AC input end (after the fuse) to protect the power supply (usually 07D471K is commonly used). However, if you have price considerations, you can ignore it for now.
●CY1, CY2 (Y-Cap):
Y-Cap can be generally divided into Y1 and Y2 capacitors. If the AC Input has FG (3 Pin), Y2-Cap is generally used. If the AC Input is 2Pin (only L, N), Y1-Cap is generally used. The difference between Y1 and Y2 is not only the price (Y1 is more expensive), but also the insulation level and withstand voltage (Y1 is called double insulation, the insulation withstand voltage is about twice that of Y2, and there will be a "return" symbol or Y1 on the body of the capacitor). This circuit uses Y2-Cap because of FG. Y-Cap will affect EMI characteristics. Generally speaking, the larger the better, but leakage and price issues must be considered. Leakage Current must meet safety requirements (3Pin company standard is 750uA max).
CX1(X-Cap), RX1:
X-Cap is an EMI prevention component. EMI can be divided into two parts: Conduction and Radiation. Conduction specifications can generally be divided into two types: FCC Part 15J Class B and CISPR 22 (EN55022) Class B. The FCC test frequency is 450K~30MHz, and the CISPR 22 test frequency is 150K~30MHz. Conduction can be verified in the factory with a spectrum analyzer, while Radiation must be verified in the laboratory. X-Cap is generally effective for EMI prevention in the low frequency band (between 150K and several M). Generally speaking, the larger the X-Cap, the better the EMI prevention effect (but the higher the price). If the X-Cap is above 0.22uf (including 0.22uf), the safety regulations require a bleeder resistor (RX1, generally 1.2MΩ 1/4W).
LF1 (Common Choke):
EMI prevention components mainly affect the mid- and low-frequency bands of conduction. EMI characteristics and temperature rise must be considered simultaneously during design. For a Common Choke of the same size, the more coils there are (the thinner the wire diameter is), the better the EMI prevention effect will be, but the temperature rise may be higher.
●BD1 (rectifier diode):
The AC power is converted to DC by full-wave rectification. From the Iin value calculated by the transformer, it can be seen that as long as a 1A/600V rectifier diode is used, the withstand voltage only needs to be 600V because it is full-wave rectification.
●C1 (filter capacitor):
The size of C1 (capacitance value) can determine the Vin(min) value in the transformer calculation. The larger the capacitance, the higher the Vin(min) but also the higher the price. This part can be used to actually verify whether Vin(min) is correct in the circuit. If the AC Input range is 90V~132V (the maximum Vc1 voltage is about 190V), a capacitor with a withstand voltage of 200V can be used; if the AC Input range is 90V~264V (or 180V~264V), because the maximum Vc1 voltage is about 380V, a capacitor with a withstand voltage of 400V must be used.
●D2 (auxiliary power diode):
Rectifier diodes are usually FR105 (1A/600V) or BYT42M (1A/1000V). The main differences between the two are:
1. Different withstand voltage (the difference does not matter here)
2. VF is different (FR105=1.2V, BYT42M=1.4V)
●R10 (auxiliary power supply resistor):
It is mainly used to adjust the VCC voltage of the PWM IC. For the currently used 3843, VCC must be greater than 8.4V (at Min. Load) during design. However, in order to consider the output short circuit situation, the VCC voltage cannot be designed too high to avoid failure to protect when the output is short-circuited (or the input wattage is too large).
●C7 (filter capacitor):
The filter capacitor of the auxiliary power supply provides a more stable DC voltage for the PWM IC. Generally, a 100uf/25V capacitor is used.
●Z1(Zener diode):
When the feedback fails, the output voltage surges, and the auxiliary power supply voltage increases relatively. If there is no protection circuit at this time, parts may be damaged. If a Zener Diode is added between 3843 VCC and 3843 Pin3, when the feedback fails, the Zener Diode will collapse, causing Pin3 to reach 1V in advance, thereby limiting the output voltage and achieving the purpose of protecting parts. The value of Z1 depends on the level of the auxiliary power supply. The decision of Z1 must also consider whether it exceeds the VGS withstand voltage of Q1. In principle, the company's existing materials can be used (generally 1/2W is sufficient).
●R2 (activation resistor):
Provides the path for the first activation of 3843. During the first activation, C7 is charged through R2 to provide the voltage required by 3843 VCC. When the resistance of R2 is large, the turn-on time is longer, but the Pin wattage is smaller when short-circuited. When the resistance of R2 is small, the turn-on time is shorter, and the Pin wattage is larger when short-circuited. Generally, 220KΩ/2W MO is used.
●R4 (Line Compensation):
For high and low voltage compensation, make the 3843 Pin3 close to the same at 90V/47Hz and 264V/63Hz (generally use 750KΩ~1.5MΩ 1/4W).
●R3, C6, D1 (Snubber):
These three parts form a Snubber. The purpose of adjusting the Snubber is: 1. When Q1 is turned off, a spike will be generated. Adjusting the Snubber can ensure that the spike will not exceed the withstand voltage of Q1. 2. Adjusting the Snubber can improve EMI. Generally speaking, D1 uses 1N4007 (1A/1000V) for better EMI characteristics. R3 uses a 2W MO resistor, and the withstand voltage of C6 is based on the actual voltage difference between the two ends (generally, a ceramic capacitor with a withstand voltage of 500V is used).
Q1 (N-MOS):
Currently, 3A/600V and 6A/600V are commonly used. The RDS(ON) of 6A/600V is smaller than that of 3A/600V, so the temperature rise will be lower. If the IDS current does not exceed 3A, 3A/600V should be considered first and verified by temperature rise records. Because the price of 6A/600V is much higher than 3A/600V, the use of Q1 also needs to consider whether VDS exceeds the rated value.
●R8:
The role of R8 is to protect Q1 and prevent Q1 from being in a floating state.
●R7 (Rs resistor):
The maximum voltage of 3843 Pin3 is 1V. The size of R7 must be coordinated with R4 to achieve the purpose of high and low voltage balance. Generally, 2W MO resistor is used. When designing, R7 is determined first and then R4 is added for compensation. Generally, the voltage of 3843 Pin3 is designed to be between 0.85V and 0.95V (depending on the wattage. If the wattage is small, it cannot be too close to 1V to avoid reaching 1V due to component errors).
●R5, C3 (RC filter):
To filter out the noise of 3843 Pin3, R5 generally uses 1KΩ 1/8W, and C3 generally uses a 102P/50V ceramic capacitor. If C3 uses a smaller capacitance value, the device may not start up under heavy load (because 3843 Pin3 instantly reaches 1V); if a larger capacitance value is used, there may be problems such as not starting up under light load and the short-circuit pin is too large.
●R9 (Q1 Gate resistor):
The value of R9 will affect the EMI and temperature rise characteristics. Generally speaking, a larger resistance value means a slower turn on/off speed for Q1 and better EMI characteristics, but a higher temperature rise and lower efficiency for Q1 (mainly because the turn off speed is slower). If the resistance value is smaller, Q1 turn on/off faster, the temperature rise of Q1 is lower, the efficiency is higher, but the EMI is worse. Generally, 51Ω-150Ω 1/8W is used.
●R6, C4 (control oscillation frequency):
To determine the operating frequency of 3843, the operating frequency of R and C can be obtained from the Data Sheet. C4 is generally a 10nf capacitor (with an error of 5%), and R6 uses a precision resistor. Taking DA-14B33 as an example, C4 uses a 103P/50V PE capacitor, and R6 is a 3.74KΩ 1/8W precision resistor. The oscillation frequency is about 45 KHz.
●C5:
The function is similar to that of an RC filter. Its main function is to make high voltage and light load less prone to oscillation. Generally, 101P/50V ceramic capacitors are used.
●U1(PWM IC):
3843 is a type of PWM IC. The Duty Cycle is controlled by the feedback signal from the Photo Coupler (U2). Pin 3 has a current limiting function (maximum voltage 1V). There are two types of 3843 currently used: KA3843 (SAMSUNG) and UC3843BN (ST). The pins of the two are the same, but the oscillation frequency they generate is slightly different. UC3843BN is about 2KHz faster than KA3843. The increase in fT will lead to some problems (such as EMI problems and short circuit problems). Since KA3843 is more difficult to buy, try to use UC3843BN when designing new models.
●R1, R11, R12, C2 (primary side loop gain control):
There is an Error AMP inside the 3843. R1, R11, R12, C2 and the Error AMP form a negative feedback circuit to adjust the stability of the loop gain. Improper adjustment of the loop gain may cause oscillation or incorrect output voltage. Generally, C2 uses a vertical multilayer capacitor (with better temperature stability).
●U2 (Photo coupler)
The photocoupler mainly converts the signal from the secondary side to the primary side (in the form of current). When the TL431 on the secondary side is turned on, U2 will convert the current on the secondary side to the primary side in proportion. At this time, 3843 outputs an off signal (Low) from Pin6 (output) to turn off Q1. The reason for using a photocoupler is to meet safety requirements (the distance from primacy to secondary must be at least 5.6mm).
●R13 (secondary loop gain control):
Control the current flowing through the photo coupler. When the resistance value of R13 is small, the current flowing through the photo coupler is large, U2 conversion current is large, and the loop gain is fast (it is necessary to confirm whether it will cause oscillation). When the resistance value of R13 is large, the current flowing through the photo coupler is small, U2 conversion current is small, and the loop gain is slow. Although it is less likely to cause oscillation, it is necessary to pay attention to whether the output voltage is normal.
●U3(TL431), R15, R16, R18
Adjust the output voltage.
The output voltage cannot exceed 38V (because the maximum VKA of TL431 is 36V. If the VF value of the photo coupler is added, Vo should be safer below 38V). The Vref of TL431 is 2.5V. The purpose of connecting R15 and R16 in parallel is to fine-tune the output voltage. The value of R15 and R16 in parallel should not be too large (try to be below 2KΩ) to avoid inaccurate output.
●R14, C9 (secondary loop gain control):
Generally speaking, increasing the capacitance will slow down the gain, while decreasing the capacitance will increase the gain. The characteristics of the resistor are just the opposite of the capacitor. When the resistor is increased, the gain increases, while decreasing the resistor slows down the gain. As for the best value for gain adjustment, it can be measured using dynamic load to obtain an optimal value.
●D4 (rectifier diode):
Because the output voltage is 3.3V, and the output voltage regulator uses TL431 (Vref=2.5V) instead of TL432 (Vref=1.25V), an extra set of windings must be added to provide the power required by the Photo coupler and TL431. Because the current required by U2 and U3 is not large (about 10mA), the diode withstand voltage value of 100V is sufficient, so only 1N4148 (0.15A/100V) needs to be used.
●C8 (filter capacitor):
Because the current required by U2 and U3 is not large, only 1u/50V is needed.
●D5 (rectifier diode):
The use of output rectifier diode, D5, needs to consider:
a. Current value
b. Diode withstand voltage
For example, if the output current is 4A, a 10A diode (Schottky) should be used, but after temperature rise verification, it was found that the temperature of D5 was too high, so a 15A diode must be replaced because the VF of 10A is larger than that of 15A. The withstand voltage of 40V was verified to be in compliance, so a 15A/40V Schottky was used in the end.
●C10, R17 (secondary side snubber):
D5 will generate a spike at the moment of cutoff. If the spike exceeds the withstand voltage of the diode (D5), the diode may be in danger of breakdown. Adjusting the snubber can appropriately reduce the voltage value of the spike. In addition to protecting the diode, it can also improve EMI. R17 generally uses a 1/2W resistor and C10 generally uses a 500V ceramic capacitor. During the snubber adjustment process (264V/63Hz), pay attention to whether R17 and C10 will overheat. This situation should be avoided.
●C11, C13 (filter capacitor):
The first-stage filter capacitor on the secondary side should use a capacitor with a smaller internal resistance (LXZ, YXA...). The appropriateness of the capacitor selection can be determined based on the following three points:
a. Output ripple voltage is in compliance with the specification
b. Whether the capacitor temperature exceeds the rated value
c. Whether the voltage across the capacitor exceeds the rated value
●R19 (dummy load):
Proper use of dummy loads can make the circuit more stable, but the resistance of the dummy load cannot be too small, otherwise it will affect the efficiency. When using it, you must also pay attention to whether it exceeds the rated value of the resistor (generally, the design only uses half of the rated wattage).
●L3, C12 (LC filter circuit):
The LC filter circuit is the second-stage filter. Under the condition that the line stability is not affected, L3 is generally enlarged (with a larger inductance), so that a smaller capacitance value can be used for C12.
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