AN10876_15 Datasheet, PDF (21/29 Pages) NXP Semiconductors – Buck converter for SSL applications

NXP Semiconductors
AN10876
Buck converter for SSL applications
The dissipation increases with the switching time. When using valley detection, these
losses are reduced at switch-on but they are still present at switch-off.
5.4 Freewheel diode losses
The freewheeling diode has two loss mechanisms, forward losses and the reverse charge
losses. The forward, or conductive, losses can be estimated using the time with respect to
current and voltage drop given in Equation 40: and Equation 41.
P_f = I_led  V_f  t2  f
(40)
P_rev
=
1_--
2

V_I2

C_rev

f
(41)
Example: If f = 89.6 kHz, I_led = 0.7 A, t2 = 5.28 s, V_f = 0.7 V, V_I = 200 V and C_rev = 10 pF
then P_f = 230 mW and P_rev = 18 mW.
The forward voltage of the diode can be lowered using a Schottky diode, but these diodes
are often difficult to obtain with reverse voltages above 100 V. Care should also be taken
not to oversize this diode, as it does not appreciably lower the forward losses and the
reverse charge is often directly related to the maximum current rating of the diode.
5.5 Inductor losses
The inductor has several loss mechanisms. The calculation of these losses is very
complex and there is much debate on the way these losses contribute to the total inductor
losses. Section 5.5 simply illustrates a number of the loss mechanisms within the inductor.
5.5.1 Resistive losses
The cause of resistive losses is a combination of wire length and its thickness. The
calculation of the resistance and the losses can be derived from Equation 42 and
Equation 43:
R_DC
=
  -¹--
A
(42)
t_sw 2
P_DC
=
--¹---- 
t_sw
^I
 R_DC dt
=
1_--
3

I_p2

R_DC
(43)
0
Example: For a wire length of 1 m with a diameter of 0.56 mm:
If _Cu = 17.2  10^9, A =   R² (= 0.246 x 10^6), R_DC= 70 m and I_p = 1.48 A, then
P_DC = 51 mW.
AN10876
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 23 June 2011
© NXP B.V. 2011. All rights reserved.
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