2 STD1LNK60Z-based RCC Control Circuit
ST公司自激式开关电源设计二
Components
2.1 MOSFET
The STD1LNK60Z (see Appendix A: STD1LNK60Z-based RCC Circuit Schematics on
page 22) has built-in, back-to-back Zener diodes specifically designed to enhance not only the Electrostatic Discharge (ESD) protection capability, but also to allow for possible voltage transients (that may occasionally be applied from gate to source) to be safely absorbed.
2.2 R3 Startup Resistor
2.2.1 Minimum Power Dissipation
The startup resistor R3 is limited by its power dissipation because of the high input bus voltage that moves across it at all times. However, the lower the R3 value is, the faster the startup
speed is. Its power dissipation should be less than 1% of the converter’s maximum output power. The minimum power dissipation value is expressed as
2.2.2 Maximum Power Dissipation
If R3 is set to 4.2M , its max power dissipation is expressed as
2.2.3 Startup Resistors and the Power Margin
The power rating for an SMD resistor with a footprint of 0805 is 0.125W. Three resistors
(1.2M , 1.2M , and 1.8M , respectively) are placed in series to produce the required startup resistor value and still have enough power margin.
2.3 Optocoupler Power Methods
There are two methods for powering the optocoupler:
l fly-back (see Figure 2), and
l forward (see Figure 3).
The fly-back method was chosen for the RCC application because it provides more stable power for the optocoupler.
Figure 2. Optocoupler Fly-back Power
Figure 3. Optocoupler Forward Power
2.4 R7 Sense Resistor
2.4.1 Minimum Power Dissipation
Sense resistor R7 is used to detect primary peak current. It is limited by its maximum power dissipation, which is set to 0.1% of the maximum power. The minimum power dissipation is expressed as
2.4.2 Maximum Power Dissipation
If R7 is set to 3.4 , its maximum power dissipation is expressed as
2.4.3 Sense Resistors and the Power Margin
Two resistors (6.8 , and 6.8 , respectively) are placed in parallel to produce the required sense resistor value and still have enough power margin.
Ramp-up voltage (via R7 x Ippk), when added to the DC voltage [(I1+Ie)(R7+R9)] achieves good output voltage and current regulation (see Figure 4).
Note: The R9 value should be much greater than the R7 value. The minimum primary current, Ippk, and the maximum current, I2, are in a stead state at the minimum load, while the maximum Ippk and the minimum I2 are in a stead state at the maximum load.
The cathode current, Ik, of TL431 is limited to 1mA< Ik <100mA, and the maximum diode current of optocoupler PC817 is 50mA. In order to decrease quiescent power dissipation, the maximum operation diode current, IF, of PC817 can be set to 10mA.
The Current Transfer Ratio (CTR) of PC817 is about 1:0 at the stead state. As a result, the maximum operation transistor current Ie of PC817 is also set to 10mA. Initially the effect of I1 is neglected.
At minimum load,
VQbe = Cut off voltage; when the voltage between the base and the emitter of transistor Q2
reaches this value, MOSFET Q1 is turned off.
For the purposes of this application design: R9 = 360 , and
C6 = 2.2nF; the role of C6 is to accelerate the MOSFET’s turning OFF.
Figure 4. Current Sense Circuit
2.5 Constant Power Control
The pole of capacitor C7 can filter the leading edge current spike and avoid a Q2 switch malfunction. However, it will also lead to delays in primary peak transfer as well as the turning on of Q2. As a result, different power inputs are produced at different input voltages.
Z1, R11, and R11a provide constant current, which is proportional to the input voltage. This way, power inputs are basically the same at different input voltages.
Note: They must be carefully selected and adjusted to achieve basically constant power input
at different input voltages. The basic selection process is expressed as
where,
I = Current change
VDC = Input bus voltage Lp = Primary Inductance Td = Transfer delay
In relation to the present RCC application,
Na = Auxiliary Winding Turns Np = Primary Winding Turns Vo = Optocoupler voltage
VF = Fly-back voltage
Ns = Secondary Winding Turns
Vz1 = Zener diode 1 voltage
Note: R11>> R9 >> R7, so in this case, only R11 is used:
Note: Constant control accuracy is not as good if Z1 is not used, and applying it is very simple.
For the purposes of this application design: C7 = 4.7nF, and
R11 = 36K
2.6 Zero Current Sense
C5 blocks DC current during starting up and allow charge to be delivered from the input voltage through starting up resistor until MOSFET turns on for the first time. The MOSFET C5 and input capacitor Ciss form a voltage divider at the MOSFET gate, so C5 value should be ten times
more than that of Ciss. This decreases the MOSFET (full) turn-on delay. In this case, C5 =
6.8nF.
R10 limits power dissipation of zener diode inside the MOSFET. The selection process is expressed as
where,
VDC(max) = Maximum input bus voltage
Na = Auxiliary Winding Turns Np = Primary Winding Turns Vo = Optocoupler voltage
VF = Fly-back voltage
Ns = Secondary Winding Turns
VZD = Zener diode voltage
IZD = Zener diode current
10mA, the value of R10 is: R10 = 1.5K
R12 limits current Ie of PC817, so the value of R12 is: R12 = 1K
2.7 Constant Voltage And Constant Current
l The Constant Voltage (CV) configuration is comprised of the error amplifier TL431, R21, R22, and C11. TL431 provides the reference voltage. R21 and R22 divide the output voltage and compare it with the reference. C11 compensates the error amplifier TL431.
R19 limits the optocoupler diode current IF (see Figure 5 and Figure 6 on page 18 for operation characteristics).
For the purposes of this application, the devices selected are: R21=1k ;
R22=1k ;
C11=100nF; and
R19=150 .
l The Constant Current (CC) can be established simply with a transistor, Q3, R16, R18,
R15, and C10. Output current flows through the sense resistor R16. Q3 is turned on when the voltage drop of R16 reaches the same value as the base turn-on voltage of Q3. This increases the current through the optocoupler and the converter goes into constant current regulation.
R16 senses the output current, and R18 limits the base current of Q3. The rating power of
R16 must then be considered. If Io = 0.4A and Vb = 0.5V, then
Two resistors, one 3.0 and one 2.2 , with SMD1206 footprint are placed in parallel to get the
required power dissipation and resistance value.
Similarly, R15 limits the optocoupler’s IF diode current for constant current regulation. C10
compensates the constant current control.
For the purposes of this application, the devices are: R15 = 75 ,
R18 = 360 , and
C10 = 1nF.
Note: The parameters of the remaining transformer devices can be seen in the Bill of Materials
(BOM, see Appendix B: STD1LNK60Z-based RCC Circuit Bill of Materials).
Figure 5. CV and CC Curve at 110VAC
Note: VDS = 200V/div; time = 4μs/div)
Figure 6. CV and CC Curve at 220VAC
