HA16107P Datasheet, PDF (36/40 Pages) Hitachi Semiconductor – PWM Switching Regulator for High-performance Voltage Mode Control

HA16107P/FP, HA16108P/FP
The reason for this is that, when the switch element is on in each cycle, there is an impulse current associated with
charging of transformer floating capacitance C_X, and IC current detection malfunctions (see Figure 7).
V₁₁
V₁₂
V_TH
Figure 7 Current Detection Waveform
<Setting Numeric Values>
If the switch element current to be detected is designated I_D, and the current detection resistance R_CS, then the following
equation is satisfied using the parameters in Figure 6.
I_D × R_CS = ((R_A + R_B)/R_B) V_TH
V_TH is the detection level voltage of the IC (240 mV in the case of the HA16107, for example). R_A and R_B are set to
values on the order of several hundred Ω to several kΩ, so that R_CS is not affected.
Next, the filter cutoff frequency is set according to the following equation.
f_C = 1/(2π C_A (R_A/R_B))
f_C can be found with the following guideline, using IC operating frequency f_OSC, power supply rating on-duty D, and
power MOS element turn-on time t_ON.
fosc/D ≤ f_C ≤ 1/(100 × t_ON)
Value 100 in the above equation provides a margin for noise, ringing, and so forth.
<Actual Example>
In an SW power supply using an HA16107, with a 100 kHz operating frequency and a D value of 30%, the relevant
values were as follows: V_B = 140 V, C_X = 80 pF, t_ON = 10 ns. Thus, when R_CS = 1 Ω, the V₁₁ level peak value reaches
the following figure.
V₁₁ (peak) = R_CS × I_D peak
= R_CS × (V_B × C_X)/t_ON
= 1Ω × (140 V × 80 pF)/10 ns
= 1.12 (V)
A filter with the following constants was then inserted.
R_A = R_B = 1 kΩ, C_A = 1000 pF
At this time, the detectable drain current is 0.48 (A), and the filter cutoff frequency is 318 (kHz). Note that increasing a
filter time constant is effective against noise, but if the value is too large, error will arise in the switch element current
detection level.
Rev.4.00 Jun 15, 2005 page 36 of 39