“First, let’s briefly explain how this part works. On the secondary side of the transformer T1, the electrical energy generated by the primary-side MOSFET switch (ON/OFF) is transmitted through the insulating barrier layer. Since the AC voltage is repeatedly ON/OFF, in order to convert it into the necessary DC voltage, diode rectification is performed through a diode D6, and it is successfully converted into DC. Since the rectified voltage has ripples, the output capacitors C7 and C8 are used to smooth the ripples and convert them into a DC voltage with small ripples.
This section will describe the rectifier diode D6 and output capacitors (Cout) C7 and C8 that are placed on the secondary side of the transformer T1.
First, let’s briefly explain how this part works. On the secondary side of the transformer T1, the electrical energy generated by the primary-side MOSFET switch (ON/OFF) is transmitted through the insulating barrier layer. Since the AC voltage is repeatedly ON/OFF, in order to convert it into the necessary DC voltage, diode rectification is performed through a diode D6, and it is successfully converted into DC. Since the rectified voltage has ripples, the output capacitors C7 and C8 are used to smooth the ripples and convert them into a DC voltage with small ripples.
As far as the overall current flow is concerned, as explained in the “Basics of Insulated Flyback Converter: Switching AC/DC Conversion” project, the input voltage from the AC power supply is rectified by bridge diodes and temporarily converted to DC Voltage. The DC voltage is then divided (chopped) by the MOSFET controlled by the switching power supply IC to produce AC voltage, and then converted into the desired DC power through the rectification of the output stage and the smoothing circuit. This design In the middle, it is converted to 12VDC. For the overall circuit, please refer to the “Isolated Flyback Converter Circuit Design” item.
Output rectifier diode D6
As mentioned above, the AC voltage generated by the secondary side of the D6 rectifier transformer T1 is converted into a DC output. As shown in the circuit diagram, it is the same as a DC/DC converter that uses diode rectification (asynchronous). The different part has only the DC voltage on the primary side, which is a high voltage of hundreds of volts.
Output rectifier diode, in order to reduce loss, use high-speed diode (Schottky diode, fast recovery diode). If you use ordinary diodes, you can’t get the desired power performance, and at worst, it may cause damage due to heat. Basically, it is the same as selecting diodes for diode-rectified DC/DC converters.
Considering the margin, choose 87V÷0.7=124V ⇒ 200V specification product
The loss of the diode (approximate value) is: Pd=VF×Iout=1V×3A=3W
Generally, it is recommended to use less than 70% of the voltage margin and less than 50% of the current. In the example circuit, ROHM’s fast recovery diode RF1001T2D (200V 10A, TO-220F package) is used.
Finally, after assembling to the circuit state, confirm the temperature rise, re-discuss the components if necessary, and install heat sinks to help dissipate heat.
Output capacitor (Cout) C7, C8
In addition to rectifying and smoothing the voltage ripple, the output capacitor can also maintain the stability of the circuit during transient overtime of the load current.
When the MOSFET on the primary side is ON, current flows through diode D6 (OFF), and the output capacitor supplies current to the load. When the MOSFET is OFF, the diode D6 is turned on (ON), and the output capacitors C7 and C8 are charged and supply current to the load.
The output capacitor is determined according to the allowable peak-to-peak ripple voltage (ΔVpp) and ripple current (Is(rms)) of the powered equipment.
Generally, it is stipulated that the impedance of electrolytic capacitors (low impedance products) for switching power supplies is 100kHz, so
In other words, when the ripple voltage is allowed to be 200mVpp, a capacitor with an impedance of 0.01Ω or less must be selected.
Next, calculate the ripple current, and explore the capacitor’s rated ripple current value based on the ripple current value.
The withstand voltage is based on twice the output voltage. Vout×2=12V×2=24V ⇒ 25V or more
In the example circuit, two low-impedance 35V 1000μF capacitors for switching power supply are connected in parallel.
The output capacitor generally uses an electrolytic capacitor. Electrolytic capacitors are components with limited life. The more ripple current flows, the shorter the life. The relevant life expectancy is provided by the capacitor manufacturer on the calculation method and regulations, please confirm with the capacitor manufacturer.
The output ripple voltage and ripple current must be confirmed in the actual circuit.