Switching power supply design principle and whole process

Switching power supply is a kind of power supply that uses modern power electronics technology to control the ratio of time when the switch is turned on and off, and maintains a stable output voltage. The switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET. Compared with linear power supplies, switching power supplies increase in cost as output power increases, but the growth rates vary. The linear power supply cost is higher than the switching power supply at a certain output power point, which is called the cost reversal point. With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating. This cost reversal point is increasingly moving to the low output power end, which provides a broad space for development of switching power supplies.

The power source is like the heart of the human body and is the power of all electrical equipment. But the power supply is not as single as the heart. Because the parameters of the marked power supply characteristics include power, power supply, frequency, noise, and changes in parameters during loading; etc.; under the same parameter requirements, there are indicators such as volume, weight, shape, efficiency, reliability, etc. To "make" and perfect power, so the form of the power supply is extremely large.

With the rapid development of power electronics technology, the relationship between power electronic equipment and people's work and life is increasingly close, and electronic equipment is inseparable from reliable power supply. In the 1980s, computer power supply fully realized switching power supply, and completed the computer first. In the 1990s, switching power supplies have entered various fields of electronics and electrical equipment. Program-controlled switches, communications, electronic testing equipment power supplies, control equipment power supplies, etc. have widely used switching power supplies, which has further promoted the rapid development of switching power supply technology. . The switching power supply is a kind of power supply that uses a modern power electronic technology to control the ratio of the time when the switching transistor is turned on and off, and maintains a stable output voltage. The switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET. Compared with linear power supplies, switching power supplies increase in cost as output power increases, but the growth rates vary. The linear power supply cost is at a certain output power point, but higher than the switching power supply, this cost reversal point. With the development and innovation of power electronics technology, switching power supply technology is constantly innovating. This cost reversal point is increasingly moving to the low output power end, which provides a wide space for development of switching power supplies.

Generally, electricity must be converted to meet the needs of use. Examples of conversions are: alternating current to direct current, high voltage to low voltage, high power to low power, and so on.

The working principle of the switching power supply is:

1. The AC power input is rectified and filtered into DC;

2. The switching transistor is controlled by a high frequency PWM (Pulse Width Modulation) signal, and that DC is applied to the primary of the switching transformer;

3. The secondary of the switching transformer induces a high-frequency voltage, which is supplied to the load through rectification and filtering;

4. The output part is fed back to the control circuit through a certain circuit to control the PWM duty cycle to achieve the purpose of stable output.

Switching power supply design process

1. Purpose

I hope that in a short space, I will introduce the current design process of the company. If there is any inappropriate introduction, please do not hesitate to advise.

2 Design steps:

2.1 Drawing circuit diagrams, PCB Layout.

2.2 Transformer calculation.

2.3 Parts selection.

2.4 Design verification.

3 Introduction to the design process (take DA-14B33 as an example):

3.1 Line diagram, PCB Layout, please refer to the description in the asset library.

3.2 Transformer calculation:

The transformer is an important core of the entire power supply, so the calculation and verification of the transformer is very important. The following is an introduction to the DA-14B33 transformer.

3.2.1 Determine the material and size of the transformer:

Calculation formula based on transformer

B(max) = core saturation magnetic flux (Gauss)

Lp = primary side inductance value (uH)

Ip = primary side peak current (A)

Np = primary side (main coil) turns

Ae = core cross-sectional area (cm2)

B(max) is determined by the material of the core and its own temperature. Take the TDK Ferrite Core PC40 as an example. The B(max) at 100 °C is 3900 Gauss. The design should take into account the part error, so generally take 3000~3500 Gauss. If the designed power is Adapter (with outer casing), it should be about 3000 Gauss to avoid the iron core being saturated due to high temperature. Generally speaking, the larger the core size, the higher the Ae, so it can be made larger wattage. Power.

3.2.2 Determine the primary side 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), the higher the wattage of Power, but the higher the relative price.

3.2.3 Determine the diameter and number of transformers:

When the transformer is determined, the Bobbin of the transformer can be determined. According to the slot width of Bobbin, the wire diameter and the number of wires of the transformer can be determined. The current density of the wire diameter can also be calculated. The current density is generally referenced to 6A/mm2, and the current density is For the design of the transformer, it can only be used as a reference value, and the temperature rise record should be the final.

3.2.4 Decide on the duty cycle:

The Duty cycle can be determined by the following formula. The design of the duty cycle is generally based on 50%. If the duty cycle exceeds 50%, the oscillation is likely to occur.

NS = number of secondary turns

NP = number of primary turns

Vo = output voltage

VD= diode forward voltage

Vin(min) = valley voltage on the filter capacitor

D = duty cycle (Duty cycle)

3.2.5 Determine the Ip value:

Ip = primary side peak current

Iav = primary side average current

Pout = output wattage

effectiveness

PWM oscillation frequency

3.2.6 Determine the number of turns of the auxiliary power supply:

According to the circle ratio of the transformer, the number of turns and voltage of the auxiliary power supply can be determined.

3.2.7 Determine the Stress of the MOSFET and the secondary diode:

According to the circle ratio of the transformer, it can be preliminarily calculated whether the stress of the transformer conforms to the specifications of the selected parts, and the calculation is based on the input voltage of 264V (380V on the capacitor).

3.2.8 Other:

If the output voltage is 5V or less and TL431 must be used instead of TL432, consider a set of windings to provide Photo coupler and TL431.

3.2.9 Substituting the obtained data into the formula, so that B(max) can be obtained. If the B(max) value is too high or too low, the parameters must be readjusted.

3.2.10 DA-14B33 transformer calculation:

Output wattage 13.2W (3.3V/4A), Core = EI-28, roundable area (groove width) = 10mm, Margin Tape = 2.8mm (per side), remaining wrapable area = 4.4mm.

Assume fT = 45 KHz, Vin(min)=90V, ? = 0.7, PF=0.5(cosθ), Lp=1600 Uh

Calculation formula:

Transformer material and size: l

From the above assumptions, the material is PC-40, size = EI-28, Ae = 0.86 cm2, the area that can be wound (groove width) = 10 mm, and the Margin Tape uses 2.8 mm, so the remaining wrapable area is 4.4 mm.

Assume that the filter capacitor uses 47uF/400V, and Vin(min) is tentatively 90V.

Determine the wire diameter and number of wires of the transformer:

Suppose NP uses a 0.32 inch line

Current density =

Number of turns possible =

Suppose the Secondary uses a 0.35 inch line.

Current density =

Assuming 4P is used, then

Current density =

Number of turns possible =

Decide on the Dutyl cycle:

Assume Np=44T, Ns=2T, VD=0.5 (using schottky Diode)

Determine the Ip value:

Determine the number of turns of the auxiliary power supply:

Assume that auxiliary power = 12V

NA1=6.3 laps

Suppose you use a 0.23 inch line

Number of turns possible =

If NA1=6Tx2P, the auxiliary power supply = 11.4V

Determine the Stress of the MOSFET and the secondary diode:

MOSFET (Q1) = highest input voltage (380V) + =

=463.6V

Diode (D5) = output voltage (Vo) + x maximum input voltage (380V) =

=20.57V

Diode(D4)=

= =41.4V

other:

Because the output is 3.3V, and the Vref value of the TL431 is 2.5V, if the voltage drop on the photo coupler is about 1.2V, the output voltage will not be able to push the Photo coupler and the TL431, so a separate set of coils must be added to provide feedback. The voltage required for the path.

Assuming that NA2 = 4T uses a 0.35 twist line, then

The number of turns can be =, so NA2 can be set to 4Tx2P

Wiring diagram of the transformer:

3.3 Parts selection:

For the location of the part (labeled), please refer to the wiring diagram: (DA-14B33 Schematic)

3.3.1 FS1:

The Iin value is calculated by the transformer. With the Iin value (0.42A), it is known that the company's common material 2A/250V is used. When designing, the Iin when Pin(max) is considered will exceed the fuse rating.

3.3.2 TR1 (Thermistor):

At the moment of power-on, the Iin current is very large due to the short circuit of C1 (primary side filter capacitor). Although the time is very short, it may also cause damage to Power. Therefore, a thermistor must be added before the filter capacitor to limit the boot. Instant Iin is within Spec (115V/30A, 230V/60A), but the thermistor will also consume power, so do not put too much resistance (otherwise it will affect the efficiency), generally use SCK053 (3A/5Ω), If the C1 capacitor uses a large value, you must consider increasing the resistance of the thermistor (usually used on a large wattage of Power).

3.3.3 VDR1 (surge absorber):

When a lightning strike occurs, the parts may be damaged, which may affect the normal operation of the Power. Therefore, it is necessary to protect the Power (usually commonly used 07D471K) by adding a surge absorber to the AC input (after Fuse), but if there is a price The considerations can be ignored first.

3.3.4 CY1, CY2 (Y-Cap):

Y-Cap can be generally divided into Y1 and Y2 capacitors. If AC Input has FG (3 Pin), Y2-Cap is generally used. If AC Input is 2Pin (only L, N), Y1-Cap, Y1 and Y2 are generally used. In addition to the price (Y1 is more expensive), the insulation grade and withstand voltage are different (Y1 is called double insulation, the insulation withstand voltage is about twice that of Y2, and there will be a "back" symbol on the body of the capacitor or indicate Y1) This circuit uses Y2-Cap because of FG. Y-Cap will affect EMI characteristics. Generally, the bigger the better, but the leakage and price issues must be considered. Leakage Current must meet the safety requirements (3Pin standard) 750uA max).

3.3.5 CX1 (X-Cap), RX1:

X-Cap is a EMI-resistant part. EMI can be divided into two parts: Conduction and Radiation. The Conduction specification can be divided into two types: FCC Part 15J Class B and CISPR 22 (EN55022) Class B. The FCC test frequency is 450K~30MHz. CISPR 22 test frequency is 150K~30MHz, Conduction can be verified by spectrum analyzer in the factory, Radiation must be verified in the laboratory, X-Cap is generally effective for EMI control in low frequency range (between 150K and M). The larger the X-Cap, the better the EMI control effect (but the higher the price). If the X-Cap is above 0.22 uf (including 0.22 uf), the safety regulations must have a bleeder resistor (RX1, generally 1.2). MΩ 1/4W).

3.3.6 LF1 (Common Choke):

EMI control parts mainly affect the middle and low frequency bands of Conduction. The EMI characteristics and temperature rise must be considered at the same time. In the same size of Common Choke, the number of coils is larger (relative wire diameter is finer), EMI prevention The better the effect, but the temperature rise may be higher.

3.3.7 BD1 (rectifier diode):

The AC power supply is converted to DC by full-wave rectification, and the Iin value calculated by the transformer can be used as long as the rectifier diode of 1 A/600 V is used, and since it is full-wave rectification, the withstand voltage is only 600 V.

3.3.8 C1 (filter capacitor):

The size (capacitance value) of C1 can determine the value of Vin(min) in the calculation of the transformer. The larger the capacitance, the higher the Vin(min) but the higher the price. This part can actually verify whether Vin(min) is in the circuit. Correct, if the AC Input range is 90V~132V (Vc1 voltage is up to about 190V), you can use a capacitor with 200V; if the AC Input range is 90V~264V (or 180V~264V), the Vc1 voltage is about 380V, so it must be Use a capacitor with a withstand voltage of 400V.

Re: Switching power design

3.3.9 D2 (auxiliary power supply diode):

Rectifier diodes, commonly used FR105 (1A/600V) or BYT42M (1A/1000V), the main difference:

1. Different withstand voltage (it does not matter if you use the difference here)

2. VF is different (FR105=1.2V, BYT42M=1.4V)

3.3.10 R10 (auxiliary power supply resistor):

Mainly used to adjust the VCC voltage of the PWM IC. In the current 3843, the VCC must be greater than 8.4V (Min. Load) when designing, but in order to consider the output short circuit, the VCC voltage cannot be designed too high, so as not to be The output is not protected when it is shorted (or the input wattage is too large).

3.3.11 C7 (filter capacitor):

The filter capacitor of the auxiliary power supply provides a stable DC voltage of the PWM IC, and generally uses a 100uf/25V capacitor.