SG1842 查看數據表（PDF） - Microsemi Corporation

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SG1842

Current Mode PWM Controller

Microsemi Corporation 

Current Mode PWM Controller

A set of formulas are given to determine the values of RT and CT for a given frequency and maximum duty

cycle. (Note: These formulas are less accurate for smaller duty cycles or higher frequencies. This will require

trimming of RT or CT to correct for this error.)

Example:

A Flyback power supply requires a maximum of 45% duty cycle at a switching frequency of 50 kHz. What

are the values of RT and CT?

Given:

f = 50kHz

Dm = 0.45

1

Calculate: RT = 267 �(1.76)..4555 − 1� = 674 Ω

(1.76).45− 1

1.86 ∗ 0.45

CT = 50000 ∗ 674 = .025 (µF)

1000

100

10

R

T

RT

=

=

680Ω

2kΩ

RT = 5kΩ

R

T

=

10kΩ

RRRRRTTTTT=====3257100000kkkk0ΩΩΩΩkΩ

1

.001

.002

.005

.01

.05

0.1

CT Value - (µF)

Figure 16 · Oscillator Frequency Vs. RT For Various CT

12

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