HV9921 datasheet(8/19 Pages) MICROCHIP

8 / 19 page

HV9921/HV9922/HV9923

DS20005311A-page 8

 2014 Microchip Technology Inc.

FIGURE 4-1:

CONDUCTION LOSS

COEFFICIENTS KC AND Kd

4.3

EMI Filter

As with all off-line converters, selecting an input filter is

critical to obtaining good EMI. A switching side capaci-

tor, albeit of small value, is necessary in order to ensure

low impedance to the high frequency switching cur-

rents of the converter. As a rule of thumb, this capacitor

should be approximately 0.1-0.2 μF/W of LED output

power. A recommended input filter is shown in

Figure 4-2 for the following design example.

4.3.1

DESIGN EXAMPLE

The following example designs a HV9921 LED lamp

driver meeting the following specifications:

• Input: Universal AC, 85-265VAC

• Output Current: 20mA

• Load: String of 10 LED (LW541C by OSRAM VF

= 4.1V max. each)

4.3.1.1

Step 1. Calculating L1.

The output voltage VO = 10 x VF ≈ 41V (max.). Use this

equation assuming a 30% peak-to-peak ripple.

Select L1 68mH, I = 30mA. Typical SRF = 170KHz.

Calculate the coil capacitance.

4.3.1.2

Step 2. Selecting D1

Usually, the reverse recovery characteristics of ultra-

fast rectifiers at IF = 20 ~ 50mA are not provided in the

manufacturer’s data books. The designer may want to

experiment with different diodes to achieve the best

result.

Select D1 MUR160 with VR = 600V, trr ≈ 20ns (IF =

20mA, IRR = 100mA) and CJ ≈ 8pF (VF > 50V).

4.3.1.3

Step 3. Calculating total parasitic

capacitance

4.3.1.4

Step 4. Calculating the leading edge

spike duration

4.3.1.5

Step 5. Estimating power dissipation

in HV9921 at 265VAC

Switching power loss:

Minimum duty ratio:

Conduction power loss:

Total power dissipation in HV9921:

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Kd(Dm)

Kc(Dm)

L1 41V 10.5s



0.3 20mA



---------------------------------- 72mH

 SRF







------------------------------------------

68mH

 170KHz







------------------------------------------------------------- 13pF

Cp 5pF 5pF 13pF 8pF 13pF

TSPIKE

264V

2 31pF



100mA

-------------------------------------------- 20ns 136ns TBLANK MIN







PSWITCH

2 10.5

s



-------------------------- 264V 31pF 2 100mA 20ns





264V 41V

–







131mW



41V

265V



-------------------------- 0.11



PCOND 0.25 20mA





210





0.63 200

A 264V

55mW





PTOTAL 131mW 55mW 186mW

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