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ASTEC公司AS431应用

ASTEC公司AS431应用

Secondary Side Error Amplifier Using the AS431

Mike Wong

I. Introduction

One of the most important safety regulations to which an off-line power supply must conform is input to output electrical isolation. This isolation requirement prevents the power supply control IC from directly sensing both the input line and output  voltages.  In  the  case  of  primary  side control  the  output  regulation  information,  an error voltage, must be transferred from the sec- ondary side. This application note discusses a simple way of transmitting regulation information across the electrical isolation using an AS431 and a conventional 4N27 opto-coupler.

II.   Power Supply Circuit

Figure 1 illustrates a simple flyback regulator. The  AS3842, a low-cost current mode control IC, is configured to regulate the power supply from  the  primary  side.  The  AS431  acts  as  a  reference and a feedback error amplifier to sense the output voltage and generate a corresponding  error  voltage.  This  error  voltage  is  then converted to an error current and coupled to the primary side through a 4N27 opto-coupler.

 

III.               Opto-Coupler

Recently,  opto-coupler  manufacturers  have made major improvements in opto-coupler processing and packaging technologies, resulting in tighter current transfer ratio (CTR) tolerances and better long-term reliability.

 

When designing the opto-coupler feedback circuitry, the designer should note the opto-coupler forward diode current. The forward diode current sets the device’s CTR and effects the longterm reliability of the device. Similar to a lamp filament, the opto-coupler diode can be worn out or  degraded  more  quickly  if  it  is  subjected  to higher  current.  Also,  the  opto-coupler’s  unity gain  bandwidth  increases  with  forward  diode current. The modulation of the gain bandwidth is caused by variations in the transconductance of the output transistor. In addition, the Miller ca- pacitor from the base to collector of the output transistor  damps  out  the  effects  of  the  opto- coupler’s  gain  variance.  A  properly  designed opto-coupler  circuit  not  only  increases  long- term reliability of the regulator but also ensures a superior loop response.

     

IV.  Design Example

Figure 2 shows the amplifier feedback section of the flyback power supply. To keep the 5 V output regulated,  the  VCOMP   voltage  must  track  the output voltage. The output voltage is first divided down by two 2.5 kW resistors, and its result is fed into an AS431 error amplifier network. The error amplifier output, VCATHODE, is then converted to a proportional opto-coupler diode current. The opto-coupler  bridges  the  isolation  barrier  and generates  an  output  collector  current  propor- tional to the input diode current. Since the opto- coupler output is connected to the VCOMP pin, the opto-coupler output current is the ICOMP source current. In a normal operating condition, a higher output  voltage  causes  VCATHODE  to  drop  and results in a high diode current and ICOMP source current and consequently a lower VCOMP. A lower VCOMP decreases the PWM duty cycle and there- fore decreases the regulator output voltage. The result  is a regulated output. A determination of the  opto-coupler  diode  operating  current  and small signal loop gain follows.

 

IVa. Opto-Coupler Operating Current

This design example shows the diode operating current as determined by the maximum ICOMP source current. In order for VCOMP  to decrease linearly  with  increasing  ICOMP  source  current, ICOMP  has to operate in a linear region slightly above the maximum ICOMP source current. The linear region is depicted in Figure 3.

 

Since the ICOMP  source current is equal to the opto-coupler  output  current,  the  opto-coupler output current also modulates in the same ICOMP linear region. With a known opto-coupler output current, the input diode current, IDIODE, can then be obtained from the output current versus diode current curve on the opto-coupler data sheet. Figure  4  illustrates  the  output  current  versus diode current curve of the 4N27 opto-coupler.

The 4N27 data sheet guarantees a minimum of 0.1 CTR at 10 mA diode current.

The typical AS3842 maximum ICOMP source cur- rent is 800 mA. Using Figure 4, and assuming 0.1 CTR at 10 mA diode current, the forward diode current required to generate 800  mA of opto-coupler current is 8 mA.

 

 

   IVb. AC Gain Analysis

Once the opto-coupler diode current is deter- mined, the current limiting resistor R1 of Figure 2 can then be chosen to guarantee good output regulations and proper dynamic loop response. The  AS431  cathode  voltage,  

  

VCATHODE,  is  a function of the diode operating current, IDIODE, and the value of R1. Also, VCATHODE  must be greater than 2.5 V for proper operation.

R1 also plays a significant role in controlling the open loop gain of the ower supply. The follow- ing equations derive the small signal AC gain from 

VCATHODE to VCOMP.

                                    

At the steady state condition, VCOMP  is in the

linear regio n,

 IVc. Other Considerations

R2, a 2 kW   resistor in parallel with the opto- coupler  diode  and  R1,  provides  the  minimum

 

              operating when a minimum opto-coupler diode current  is  required.  In  addition,  a  small  filter

capacitor is placed close to the VCOMP pin of the control IC to attenuate high frequency switching noise being picked up by the metal trace from the  opto-coupler  to  the  control  IC.  Since  the location  of  the  pole  in  the  opto-coupler  small signal response varies significantly with the dc operating point of the opto-coupler, a resistor can  be  added  from  the  VREG  to  VCOMP  pin  to supply  additional  bias  current  to  stabilize  the loop.

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    2007-10-15 09:03

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