Design of switching power supply based on PQ35

1. Introduction

Since the advent of the switching power supply in the 1950s, it has been widely used in almost all electronic equipment such as computers and communication equipment due to its small size and high efficiency. It has a wide variety of types and forms, and its development trend is also towards small size, high efficiency and low cost. The 300W switching power supply introduced here is an isolated hard switch, half-bridge switching power supply. It has good efficiency and output indicators under the conditions of low voltage (14V) and large current (22A) output. Reasonable parameters and winding process design are carried out for core components (such as high-frequency transformers).

High-frequency transformers are the core energy conversion components in switching power supplies. Like ordinary power frequency transformers, they also transmit energy through magnetic coupling. However, the magnetic circuit that realizes magnetic coupling in this power transformer is not the silicon steel sheet in ordinary transformers, but a ferrite core or beryllium malloy and other magnetic materials with high magnetic permeability that work under high frequency conditions. The purpose is to obtain a larger excitation inductance and reduce the power loss in the magnetic circuit, so that it can transmit pulse energy with a wide bandwidth with minimal loss and phase distortion [1].

There are many types of ferrite cores, including EC, EE, EDT, EP, EPC, EF, EI, PQ, RM, P, LP, etc., which are used in various requirements. Among them, the PQ type core has the advantage of good magnetic shielding, which reduces the propagation of electromagnetic interference (EMI). It is particularly suitable for switching power supply transformers, and has better effects in the range of 50W to 1000W.

2. Use PQ35/35 to make a 300W half-bridge converter high-frequency switching power supply

2.1 300W (14V, 22A) half-bridge switching power supply schematic

The schematic diagram of the 300W (14V, 22A) half-bridge switching power supply is shown in Figure 1. As can be seen from the schematic diagram, the 300W half-bridge switching power supply is mainly composed of two power tubes IRFP460LC, a high-frequency transformer PQ35/35 (PC40 material), an output rectifier filter circuit, a drive circuit, a protection circuit, a control circuit (not shown in this figure due to space limitations) and an auxiliary power supply.

The principle of half-bridge switching power supply has been explained in detail in many books, so this article will not go into details. The parameter design and winding process of high-frequency transformer are explained in detail below.

2.2 Parameter design and winding of 300W half-bridge switching power supply high-frequency transformer

The magnetizing characteristics of the half-bridge high-frequency transformer work in the first and third quadrants, and the magnetic flux changes from -Bm to +Bm. It is a symmetrical working transformer and does not require an air gap.

(1) Estimation of the power capacity of the PQ35 core

The power capacity calculation formula of the switching power supply transformer is [2]:

In the formula

PT--apparent power of transformer, for half-bridge type PT=(1/η+1.4 1 4)P. , where efficiency η=8 5%, Po=300W, then PT=777w;

Ko--the copper filling factor of the window, take 0.5:

Kf--waveform coefficient, 4 for square wave;

Fw--switching frequency, in this case 100KHz:

Bw - core working flux density, generally 1/3 of the saturation flux density. This example uses PC 4 0 material, saturation flux density G s = "0". 39T (1 00 ℃), take Bw = 0.1 3T:

Kj - current density proportionality coefficient, generally taken as 400:

X--constant, determined by the selected magnetic core, can be -0.12.

Then AP=(7 7 7*1 04/0.5*4*1 00*1 03*0.1 3*4 00)1.14≈0.72cm4.

It can also be calculated by the following formula [3]:

In the formula

Pt - nominal output power of the transformer, which can be 450W for 1.5Po;

η--Transformer efficiency, here we take 85%:

f--switching frequency, in this case 100KH z:

Bm--The maximum magnetic induction intensity of the core, take 1500GS:

δ--The current density of the enameled wire, 100KHz frequency can be 6A/mm2:

Km - copper filling factor of the window, take 0.5:

Kc--core filling factor, can be 1.0.

则A P="4" 5 0*1 06/(2*0.8 5*1 00*1 03*1 5 00*4*0.5*1)≈0.59cm4。

Looking up the table PQ 3 5/3 5, AP="4".3 cm4 far exceeds the power capacity requirement. The reason for choosing PQ35 is to prepare for increasing the output power in the future.

(2) Calculate the number of turns of the primary winding

The number of turns of the primary winding can be calculated by the following formula:

In the formula

Kf--waveform coefficient, square wave

Pt - nominal output power of transformer, 1.5Po can be taken as 4sow;

Fw - switching frequency, in this case η1 - efficiency of the transformer, here 1ooKHz:

Bw--core working magnetic flux density, take 0.1 3T;

Ae--Core effective area, PQ35/35 is 196mm2;

V--transformer primary voltage, in this example V=250*1.3/2=1 62.5V.

Then Np=1 6 2.5/4*1 00*1 08*0.1 3*1 9 6*1 0-6≈ 1 5.9, round up to 1 6 turns. Later, after experiments, 20 turns were found to be more appropriate.