HF900 – 900V OFFLINE SWITCHING REGULATOR

HF900 Rev. 1.0

www.MonolithicPower.com

16

8/4/2015

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APPLICATION INFORMATION

Selecting the Input Capacitor

The bulk capacitors of the rectifier bridge filter the

rectified AC input, which supplies the DC input

voltage for the converter. Figure 6 shows the

typical DC bus voltage waveform of a full-bridge

rectifier.

t

DC input voltage

AC input voltage

0

t1

Figure 6: Input Voltage Waveform

When the full-bridge rectifier is used, usually the

input capacitor is set at 2μF/W for the universal

input (>185VAC) application, cut the capacitor

with Equation (3):

OO

in

VI

P

×

=

η

(3)

output current, and

η is the estimated efficiency.

Generally, η is between 0.75 and 0.85 depending

on the input range and output application.

From the waveform in Figure 6, the AC input

calculated using Equation (4):

2

in

DC

AC

AC

in

2P

V(V ,t)

2 V

t

C

×

=×

−

×

(4)

DC bus voltage reaches the minimum value

using Equation (5):

DC(min)

DC

AC(min)

VV (V

,t1)

=

(5)

Very low DC input voltage can cause a thermal

problem in a full load. It is recommended that the

minimum DC voltage is higher than 70V.

Otherwise the input capacitor value should be

increased.

As a 900V offline regulator, the HF900 suits very

high-voltage input applications. General input

capacitors with 400V voltage ratings cannot

satisfy the safety requirement. Thus, stack

capacitors can be used in very high input voltage

applications such as a 420VAC input (see Figure

7).

R2

R3

R4

R1

C2

D1

D2

D3

D4

C1

85~420VAC

Bus voltage

Figure 7: Input Stack Capacitor Circuit

C1 and C2 endure half of the input DC voltage

rating, respectively. R1 to R4 should use the

same value resistor to equalize the C1 and C2

voltage stress. It is recommended to use a 1206

package for R1 to R4 to satisfy the safety

requirement. Also, the R1 to R4 values should be

large enough for energy saving. For example, the

total value of R1 to R4 is 20MΩ, which consumes

about 18mW in 600VDC bus voltage.

Normally, the converter is designed to operate in

CCM with low input voltage. CCM is needed to

satisfy the output energy requirement for the

universal input condition. With a built-in slope

compensation function, the HF900 supports CCM

when the duty cycle exceeds 50%. Set the ratio

relevant waveforms. A larger inductor leads to a

increases

the

transformer

size.

For

5W