开关模式电源供应器(SMPS)设计指南

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"Switch Power Mode Spply - 高清晰PDF文档，涵盖了开关电源技术，包括开关模式电源的应用、功率半导体的选择指南、输出整流、磁性元件设计以及谐振电源供应等内容。由Philips Semiconductors提供，深入探讨了开关模式电源在各种拓扑结构中的使用。" 开关电源（Switched Mode Power Supply，简称SMPS）是一种高效能的电力转换设备，与传统的线性电源相比，具有显著的优点。传统的线性电源由于包含大型、重的50/60Hz变压器和耗散式串联调节器，效率低下且体积庞大。而SMPS则利用开关半导体器件，通过高频切换来实现电源转换，从而提高了效率和功率密度。 2.1 使用功率半导体在开关模式拓扑中的应用这部分内容详细介绍了如何在不同的开关模式拓扑结构中应用功率半导体，如Buck、Boost、Buck-Boost、Flyback、Forward等，并提供了选择合适功率半导体的指南。功率半导体，如MOSFET和IGBT，是SMPS的核心组件，它们在开关过程中承担电流的开通和关断，对电源性能至关重要。 2.2 输出整流在SMPS中，输出整流是将高频交流电压转换为直流电压的关键步骤。通常采用二极管或更先进的同步整流技术，同步整流可以进一步提高系统的效率。 2.3 设计示例这部分可能包含实际的设计案例，展示如何将理论知识应用于实际电源设计中，包括电路布局、元器件选择、控制策略等，帮助读者理解和掌握SMPS的设计流程。 2.4 磁性元件设计磁性元件，如电感和变压器，是SMPS中的关键部分。它们的设计涉及到磁芯材料的选择、绕组设计、磁通密度计算等，对于电源的效率、体积和稳定性有着直接影响。 2.5 谐振电源供应谐振电源供应利用谐振电路特性，可以实现零电压开关或零电流开关，从而降低开关损耗，提高效率。这种技术适用于高频和高效率的电源设计。通过这份Philips Semiconductors提供的资料，读者可以深入了解开关电源的工作原理、设计方法以及优化技巧，对于从事电源设计或相关领域的工程师来说，是一份宝贵的参考资料。

S.M.P.S. Power Semiconductor Applications

Philips Semiconductors

Output power 100W 200W 300W

Line voltage, Vin 110V ac 220V ac 110V ac 220V ac 110V ac 220V ac

Transistor requirements

Max current 2.25A 1.2A 4A 2.5A 6A 3.3A

Max voltage 400V 800V 400V 800V 400V 800V

Bipolar transistors.

TO-220 BUT11 BUX85 BUT12 BUT11A --- BUT12A

Isolated SOT-186 BUT11F BUX85F BUT12F BUT11AF --- BUT12AF

SOT-93 --- --- --- --- BUW13 ---

Isolated SOT-199 --- --- --- --- BUW13F ---

Power MOSFET

TO-220 BUK454-400B BUK454-800A BUK455-400B BUK456-800A --- ---

Isolated SOT-186 BUK444-400B BUK444-800A BUK445-400B BUK446-800A --- ---

SOT-93 --- --- --- --- BUK437-400B BUK438-800A

Output Rectifiers (dual)

O/P voltage

5V PBYR2535CT --- ---

10V PBYR20100CT PBYR30100PT PBYR30100PT

BYV32E-100/150/200 BYV42E-100/150/200 BYV72E-100/150/200

BYV72E100/150/200

20V PBYR20100CT PBYR20100CT PBYR20100CT

BYQ28E-100/150/200 BYV32E-100/150/200 BYV32E-100/150/200

50V BYT28-300 BYT28-300 BYT28-300

Table 3. Recommended Power Semiconductors for single-ended forward.

Forward

Converter efficiency, η = 80%; Max duty cycle, D

max

= 0.45

Max transistor voltage, V

or V

= 2V

in(max)

dc voltage gain:-

Applications:- Low cost, low output ripple, multiple output supplies in the 50 to 400W range. E.g. small computer

supplies, DC/DC converters.

Maxtransistorcurrent,I

; I

out

η D

max

min

Vin

= nD

118

S.M.P.S. Power Semiconductor Applications

Philips Semiconductors

Operation.

To utilise the transformer flux swing fully, it is necessary to

operate the core symmetrically as described earlier. This

permits much smaller transformer sizes and provides

higher output powers than possible with the single ended

types. The symmetrical types always require an even

number of transistor switches. One of the best known of the

symmetrical types is the push-pull converter shown in

Fig. 10.

The primary is a centre-tapped arrangement and each

transistor switch is driven alternately, driving the

transformer in both directions. The push-pull transformer is

typically half the size of that for the single ended types,

resulting in a more compact design. This push-pull action

produces natural core resetting during each half cycle,

hence no clamp winding is required. Power is transferred

to the buck type output circuit during each transistor

conduction period. The duty ratio of each switch is usually

less than 0.45. This provides enough dead time to avoid

transistor cross conduction. The power can now be

transferred to the output for up to 90% of the switching

period, hence allowing greater throughput power than with

the single-ended types. The push-pull configuration is

normally used for output powers in the 100 to 500W range.

Fig. 10 Push-pull converter.

The bipolar switching action also means that the output

circuit is actually operated at twice the switching frequency

ofthe powertransistors, ascan be seenfromthewaveforms

in Fig. 11. Therefore, the output inductor and capacitor can

be even smaller for similar output ripple levels. Push-pull

converters are thus excellent for high power density, low

ripple outputs.

Advantages.

As stated, the push-pull offers very compact design of the

transformer and output filter, while producing very low

output ripple. So if space is a premium issue, the push-pull

could be suitable. The control of the push-pull is similar to

theforward, in that it is againbasedon the continuous mode

buck. When closing the feedback control loop,

compensation is relatively easy. For multiple outputs, the

same recommendations given for the forward converter

apply.

Clamp diodes are fitted across the transistors, as shown.

This allows leakage and magnetisation energy to be simply

channelled back to the supply, reducing stress on the

switches and slightly improving efficiency.

The emitter or source of the power transistors are both at

the same potential in the push-pull configuration, and are

normally referenced to ground. This means that simple

base drive can be used for both, and no costly isolating

drive transformer is required. (This is not so for the bridge

types which are discussed latter.)

Disadvantages.

One of the main drawbacks of the push-pull converter is

the fact that each transistor must block twice the input

voltage due to the doubling effect of the centre-tapped

primary, even though two transistors are used. This occurs

when onetransistor isoff and the other is conducting. When

both are off, each then blocks the supply voltage, this is

shown in the waveforms in Fig. 11. This means that TWO

expensive, less efficient 800 to 1000V transistors would be

required for a 220V off-line application. A selection of

transistors and rectifiers suitable for the push-pull used in

off-line applications is given in Table 4.

A further major problem with the push-pull is that it is prone

to flux symmetry imbalance. If the flux swing in each half

cycle is not exactly symmetrical, the volt-sec will not

balance and this will result in transformer saturation,

particularly for high input voltages. Symmetry imbalance

can be caused by different characteristics in the two

transistors such as storage time in a bipolar and different

on-state losses.

The centre-tap arrangement also means that extra copper

is needed for the primary, and very good coupling between

the two halves is necessary to minimise possible leakage

spikes. It should also be noted that if snubbers are used to

protect the transistors, the design must be very precise

since each tends to interact with the other. This is true for

all symmetrically driven converters.

These disadvantages usually dictate that the push-pull is

normally operated at lower voltage inputs such as 12, 28

or 48V. DC-DC converters found in the automotive and

telecommunication industries are often push-pull designs.

At these voltage levels, transformer saturation is easier to

avoid.

Since the push-pull is commonly operated with low dc

voltages, a selection guide for suitable power MOSFETs is

also included for 48 and 96V applications, seen in Table 5.

T1 D1

n : 1

TR1

TR2

Vin

119

S.M.P.S. Power Semiconductor Applications

Philips Semiconductors

(d) The Half-Bridge.

Of all the symmetrical high power converters, the

half-bridge converter shown in Fig. 12 is the most popular.

It is also referred to as the single ended push-pull, and in

principle is a balanced version of the forward converter.

Again it is a derivative of the buck. The Half-Bridge has

some key advantages over the push-pull, which usually

makes it first choice for higher power applications in the

500 to 1000W range.

Operation.

The two mains bulk capacitors C1 and C2 are connected

in series, and an artificial input voltage mid-point is

provided, shown as point A in the diagram. The two

transistorswitches are driven alternately, and this connects

each capacitor across the single primary winding each half

cycle. V

/2 is superimposed symmetrically across the

primary in a push-pull manner. Power is transferred directly

to the output on each transistor conduction time and a

maximum duty cycle of 90% is available (Some dead time

is required to prevent transistor cross-conduction.) Since

the primary is driven in both directions, (natural reset) a full

wave buck output filter (operating at twice the switching

frequency) rather than a half wave filter is implemented.

This again results in very efficient core utilisation. As can

be seen in Fig. 13, the waveforms are identical to the

push-pull, except that the voltage across the transistors is

halved. (The device current would be higher for the same

output power.)

Fig. 12 Half-Bridge converter.

Advantages.

Since both transistors are effectively in series, they never

see greater than the supply voltage, V

. When both are off,

their voltages reach an equilibrium point of V

/2. This is half

the voltage rating of the push-pull (although double the

current). This means that the half-bridge is particularly

suited to high voltage inputs, such as off-line applications.

For example, a 220V mains application can use two higher

speed, higher efficiency 450V transistors instead of the

800V types needed for a push-pull. This allows higher

frequency operation.

Another major advantage over the push-pull is that the

transformer saturation problems due to flux symmetry

imbalance are not a problem. By using a small capacitor

(less than 10µF) any dc build-up of flux in the transformer

is blocked, and only symmetrical ac is drawn from the input.

The configuration of the half-bridge allows clamp diodes to

be added across the transistors, shown as D3 and D4 in

Fig. 12. The leakage inductance and magnetisation

energies are dumped straight back into the two input

capacitors, protecting the transistors from dangerous

transients and improving overall efficiency.

A less obvious exclusive advantage of the half-bridge is

that the two series reservoir capacitors already exist, and

this makes it ideal for implementing a voltage doubling

circuit. This permits the use of either 110V /220V mains as

selectable inputs to the supply.

The bridge circuits also have the same advantages over

the single-ended types that the push-pull possesses,

including excellent transformer utilisation, very low output

ripple,andhigh outputpower capabilities. Thelimitingfactor

in themaximum output power available from the half-bridge

is the peak current handling capabilities of present day

transistors. 1000W is typically the upper power limit. For

higher output powers the four switch full bridge is normally

used.

Disadvantages.

The need for two 50/60 Hz input capacitors is a drawback

because of their large size. The top transistor must also

have isolated drive, since the gate / base is at a floating

potential. Furthermore, if snubbers are used across the

power transistors, great care must be taken in their design,

since the symmetrical action means that they will interact

with one another. The circuit cost and complexity have

clearly increased, and this must be weighed up against the

advantages gained. In many cases, this normally excludes

the use of the half-bridge at output power levels below

500W.

Suitable transistors and rectifiers for the half-bridge are

given in Table 6.

n : 1

TR1

TR2

Vin

isolated

drive

needed

122

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