MIC3832 Просмотр технического описания (PDF) - Micrel

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MIC3832

Current-Fed PWM Controllers

Micrel 

MIC3832/3833

100kHz 100W Current-Fed Converter

Refer to Figure 5.

A 5V, 20A dc-to-dc converter uses the current-fed, push-pull

configuration for increased safety and reduced size and

transformer core area.

The input is an unregulated 14V to 32Vdc supply. An

MIC2951 low-dropout regulator supplies 12V to the MIC3833.

The main PWM switching element is an IRF540, with gate

drive provided by transformer T1. The two 50% duty cycle

outputs each drive an IRF540 directly, which in turn drive

respective sides of T2’s center tapped primary. The 1N6291A

transzorb is used to protect the MOSFETs from spikes.

Current-mode control simplifies the stability analysis, with the

0.2Ω, 5W resistor being used as the sensing element. As the

maximum duty cycle at light loads is greater than 50%, the

well characterized problem of subharmonic oscillations found

when using current-mode control was evident. A ramp,

introduced at the sensing element, provides slope compen-

sation. The 10kΩ and 470kΩ divider from the oscillator (ramp

source) to the sensing element provides the proper slope. A

large resistor value from CT to CMR makes buffering unnec-

essary.

Micrel

Front-edge blanking eliminates the need for a filter network

around the sensing element and decreases the possibility of

turn-on transients that cause system instabilities.

Four inexpensive output capacitors in parallel reduce ESR to

an acceptable level of 80mΩ without adding too much size or

cost.

Error amplifier compensation uses a simple lead-lag network.

With current-mode control there is no need to compensate for

the LC filter pole.

Soft start is implemented to allow slow turn-on in the event of

a short circuit.

All magnetics were chosen to minimize losses at 100 kHz. T2

and L1 are wound using Siemen’s N87 material and T1 using

Magnetics Inc.’s P-type material. T2 and L1 use Siemen’s

EFD core and bobbin assemblies which are designed to

reduce the height/form factor of the finished supply. T1 is a

bifilar-wound toroid used as a 200kHz pulse transformer. T2

is not gapped, since a push-pull transformer has a minimum

dc current component, being a forward converter.

20V

14—32V IRF540 1N968

L1

20µH

2000

µF

10Ω

MUR1605CT

IRF540

1N6291A

2:1 T2

1/2

MBR2535CT

0.1µF

0.2Ω

5W

IRF540

1/2 MBR2535CT

1N6291A

39kΩ

0.1µF

MYLAR

+5V 20A

220µF 220µF 0.01µF

39kΩ 16V 16V CER

100Ω

3.3µF

MIC2951

1 V OUT V IN 8

2 SENSE

7

FB

3 SHDN 5V TAP 6

4 GND ERROR 5

887kΩ

1%

100pF

102kΩ

1%

12V

3.3µF

T1

1:1

0.1µF

5.1kΩ

2.2nF

5.1

kΩ 200kΩ

MIC3833

1 GND

Q 16

2 PWM

3Q

15

CT 14

RT

4

13

VDD

SYNC

5

12

5V REF CMR

6 EA – MDC/SS 11

7 EA + SHDN 10

8 EA OUT NC 9

.01µF

10kΩ

470kΩ

2nF

10kΩ

68k 68k 1µF

Figure 5: 100W Current-Fed, Push-Pull DC-to-DC Converter (Efficiency ~75%)

Magnetics Design

T1 : Magnetics Inc # 41303 – TC, P material, Primary = 26 turns 30 gauge wire, Secondary = 26 turns 30 gauge wire

T2: Seimen’s EFD25, N87 material. Primary = 20 turns 20 gauge wire, Secondary = 10 turns trifilar wound 20 gauge

wire. Both are center tapped.

L1: Seimen’s EFD20, N87 material. 13 turns 20 gauge wire. Gap for 20µH

4-144

April 1998

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