“This application note describes the diode selection process and snubber design of a high voltage flyback converter for subscriber line interface card (SLIC) applications. The key diode parameters affecting switching transients in the circuit are discussed, as well as the design of the output diode snubber circuit.
This application note describes the diode selection process and snubber design of a high voltage flyback converter for subscriber line interface card (SLIC) applications. The key diode parameters affecting switching transients in the circuit are discussed, as well as the design of the output diode snubber circuit.
For applications where speed and switching time are an important factor, it is very helpful to use components that meet the fast reaction times you need. These applications can range from small circuits soldered on PCBs to large manufacturing machines, where timelines are the make or break deal.
Recent developments in the PC and telecom markets are now extending power electronics switching frequencies from line frequencies into the MHz range. This trend has led to a corresponding growth in Electronic switching element technology, such as power rectifiers and power switches. At these switching frequencies, ultrafast power rectifier performance is very important. It requires diodes with low recovery charge with soft recovery characteristics and low forward voltage drop for fast turn-on. The purpose of this application note is to discuss diode parameters that affect circuits for designing reliable power supplies.
To illustrate the effect of diode parameters on circuit performance, this application note uses a flyback circuit using the MAX1856 as an example. The first part briefly introduces the flyback circuit used here as an example. The second section discusses the important diode parameters that affect switching transients in the circuit, the design of the output diode snubber circuit, and the contribution of rectifier conduction, switching, and reverse blocking to the total power dissipation. Manufacturers of fast rectifiers may list all or some of the parameters discussed in Part II. The third and final part discusses the performance of the four different diodes in this circuit. This shows a way to evaluate the performance of different diodes in application circuits.
MAX1856 Flyback Circuit
The MAX1856 is used here (Figure 1) in a flyback configuration to power a Subscriber Line Interface Card (SLIC) from a 12V input. -90V at 0.32A output is used for ringing function, and -30V at 0.15A output is used for talk battery.
SLIC power supply schematic.
The MAX1856 current-mode PWM controller uses a flyback configuration to generate the relatively high negative voltage required by the SLIC supply. PWM mode controllers use fixed frequency current mode operation where the duty cycle is determined by the input to output voltage ratio and the transformer turns ratio. A current-mode feedback loop adjusts the peak Inductor current based on the output error signal. The MAX1856 uses a low-side external sense resistor (R1 in Figure 1) to monitor the peak inductor current. The current sense circuit is blanked for 100ns immediately after the controller is turned on to minimize noise sensitivity. Additionally, a filter on the current sense pin (CS+) (R10 and C7 in Figure 1) improves noise immunity. This time constant should be low enough not to distort the current sense signal. In general, the maximum R10-C7 time constant should be less than 1/10 of the minimum duty cycle at which the control loop operates normally. Refer to the MAX1856 data sheet for detailed guidelines on the design flow for this circuit.
Diode Waveforms and Characteristics
Fast rectifier diodes use some variation of the pin structure. The transition from conducting to blocking state takes a finite time. This is called the diode’s reverse recovery time (trr). This can be further divided into time ta, when the carriers are removed before it can block the voltage (current through the diode reverses for a short time), and time tb, during which the diode voltage becomes negative at the rate of change dVR/dt. Increasing the injection to lower the forward voltage drop means that more charge needs to be removed from the intrinsic region before the diode can block the voltage. Therefore, this will adversely affect the reverse recovery time. Fast recovery rectifier manufacturers often try to find the best balance of these two requirements.
Reverse recovery waveforms and definitions.
Figure 2 above shows the waveform and definition of the recovery characteristics of a fast recovery rectifier. The charge stored in the intrinsic region is removed by flowing a large reverse current during time ta. At the end of this time, the junction becomes reverse biased. The reverse current at this time is defined as the peak reverse recovery current IRRM. The value of IRRM is proportional to the rate of change of forward current through zero crossing dIF/dt.
IRRM = (dIF/dt) × ta
The reverse current then decreases by recombining at the rate of dIR/dt in time tb.The amount of reverse recovery charge is given by
QRR = (IRRM × trr)/2
where trr = ta + tb
Some rectifier datasheets may define a softness factor, S, where
S = (ta/tb)
The diode voltage now goes negative at a rate proportional to dIR/dt. During diode recovery, this change in current will cause reverse voltage overshoot due to the parasitic inductance LLS in the transformer secondary.The peak reverse voltage VRRM is given by
VRRM = LLS × dIR/dt
The parasitic diode self-capacitance CD is given by
CD = (IRRM × trr)/(2 × VRRM)
This parasitic capacitance CD resonates with the parasitic inductance LLS in the secondary of the transformer, often causing noise problems in the current sense signal and application circuit. To suppress this ringing, an RC snubber can be used at the cathode of the secondary rectifier (D2) in Figure 1 (the snubber is placed on this rectifier because the output power required for this output is the most). Snubber component values R5 and C10 are given by (see Figure 1)
R5 = √ (LLS/CD) and C10 = 3 × CD or C10 = 4 × CD
Rectifier power consumption
Finally, consider the power consumption of the rectifier in different operating modes. During the on-time of the switch, energy is being accumulated and stored in the transformer. During this time, the rectifier is blocked.The blocking state loss can be expressed as
PR = IR × VR × D
where IR is the diode’s reverse leakage current, VR is the diode’s reverse voltage, and D is the duty cycle.
At the end of this cycle, the switch is closed and energy is transferred to the output.The diode now starts to conduct and the power dissipated in the diode is
PF = IF × VF × (1-D)
where IF is the forward current of the diode and VF is the forward voltage drop of the diode.
At the end of this cycle, the diode turns off and enters a blocking state.The power dissipation during the transition from the ON state to the OFF state is given by
Prec = VRRM × IRRM × 0.5 × f × tb
where IRRM is the peak reverse recovery current, VRRM is the peak reverse voltage, and f is the switching frequency.