HA17431_15 Datasheet, PDF (15/21 Pages) Renesas Technology Corp – Shunt Regulator

HA17431 Series
Design Guide for AC-DC SMPS (Switching Mode Power Supply)
1. Use of Shunt Regulator in Transformer Secondary Side Control
This example is applicable to both forward transformers and flyback transformers. A shunt regulator is used on the
secondary side as an error amplifier, and feedback to the primary side is provided via a photocoupler.
Transformer
PWM IC
HA17384
HA17385
SBD
Phototransistor
Photocoupler
V_F
Light
emitting diode
R₁
I_F
R₂
R₃
I_B
V_K
C₁ ^R5
K
REF
HA17431
A
Vref
R₄
GND
(+)
Output
V₀
(–)
Figure 2 Typical Shunt Regulator/Error Amplifier
2. Determination of External Constants for the Shunt Regulator
A. DC characteristic determination
In figure 2, R₁ and R₂ are protection resistor for the light emitting diode in the photocoupler, and R₂ is a bypass
resistor to feed I_K minimum, and these are determined as shown below. The photocoupler specification should
be obtained separately from the manufacturer. Using the parameters in figure 2, the following formulas are
obtained:
R₁ =
V₀ – V_F – V_K
I_F + I_B
, R₂ =
V_F
I_B
V_K is the HA17431 operating voltage, and is set at around 3 V, taking into account a margin for fluctuation. R₂
is the current shunt resistance for the light emitting diode, in which a bias current I_B of around 1/5 I_F flows.
Next, the output voltage can be determined by R3 and R4, and the following formula is obtained:
V₀ =
R₃ + R₄
R₄
× Vref, Vref = 2.5 V Typ
The absolute values of R₃ and R₄ are determined by the HA17431 reference input current Iref and the AC
characteristics described in the next section. The Iref value is around 3.8 μA Typ. (V version: 2 μA Typ)
B. AC characteristic determination
This refers to the determination of the gain frequency characteristic of the shunt regulator as an error amplifier.
Taking the configuration in figure 2, the error amplifier characteristic is as shown in figure 3.
G₁
f₁ f_AC
G_{2 When R5 ≠ 0}
When R₅ = 0
f₂
f_OSC Frequency f (Hz)
* f_OSC : PWM switching frequency
Figure 3 HA17431 Error Amplification Characteristic
R03DS0086EJ0400 Rev.4.00
Jan 10, 2014
Page 15 of 20