8

8&&

(/+0) =

l8

176

+ 8+0(/+0) ×

0

5

0

2 p

× &/+0 =

l208 +728 ×

1

13p

× 0.36 = 9.28

8

8&&

(/#:) =

@8

176

+ 8+0(/#:) ×

05

02A

× &/#: =

@208 +3758 × 1

13A

× 0.5 = 24.48

:

|

u

u

u

u

!

187

100

85

1

2

1

100

1

2

1

2

kHz

F

fsw

C

R

P

S

S

26

UCC24612

SLUSCM5A – AUGUST 2017 – REVISED FEBRUARY 2018

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If the magnitude of the voltage from TP2 to TP1 is less than 28 V, remove R2 that is populated on the EVM ( 20

kΩ) and set R2 to 0 to 10 ohms and remove 27-V Zener diode D1 from the board.

If TP2 to TP1 is greater than 28 V use resistor R2 to setup an averaging filter to lower the DC voltage applied to

VDD.

The RC filter formed by C2 and R2 should set the filter pole frequency to one-hundredth of the converter's

maximum switching frequency. In this example the converter's maximum switching frequency (fSW) is 85 kHz.

Note that the switching frequency will vary based on design and preference.

(6)

When the RC filter circuit is used, it is recommended that the VDD voltage should be between 4 V to 28 V to

provide enough energy and voltage to the gate driver. This range can be determined in a fixed frequency Flyback

converter with the following equations. DMAX is the maximum duty cycle of the converter and DMIN is the minimum

duty cycle of the converter. NP is the Flyback transformer (T1) primary number of turns and NS is the transformer

secondary number of turns. Please refer to Figure 23 for details.

Maximum VDD voltage (VVDD(MAX)):

(7)

Minimum VDD voltage (VVDD(MIN)):

(8)

8.2.3 Application Curves

The UCC24612EVM is used as a synchronous rectifier in both a high-side and low-side configuration in an offline

(VIN = 85 V to 265 V RMS), 20-V (VOUT), 60-W application. The primary-side controller used in this design is a

UCC28740 secondary-side regulated, variable-frequency Flyback controller that had a maximum switching

frequency of roughly 85 kHz. Please refer to the UCC28740 data sheet for further details.

8.2.3.1 Steady State Testing Low-Side Configuration

•

Snubber Components, R2 = 1.02 kΩ, R3 = 51.1Ω , C3 = 470 pF

•

CH1 = VG, CH2 = Q1 drain (TP2), CH3 = VOUT Voltage Ripple (TP3)

Figure 24. 85-VAC, 0-A Load

Figure 25. 85-VAC, 3-A Load