Datasheet LT3798 (Analog Devices) - 13

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OPERATION Switch Voltage Clamp Requirement
period, as well. Similarly, initial values can be estimated Leakage inductance of an offline transformer is high due using stated switch capacitance and transformer leakage to the extra isolation requirement. The leakage inductance inductance. Once the value of the drain node capacitance energy is not coupled to the secondary but goes into and inductance is known, a series resistor can be added the drain node of the MOSFET. This is problematic since to the snubber capacitance to dissipate power and criti- 400V and higher rated MOSFETs cannot always handle cally dampen the ringing. The equation for deriving the this energy by avalanching. Therefore the MOSFET needs optimal series resistance using the observed periods protection. A transient voltage suppressor (TVS) and diode (tPERIOD, and tPERIOD(SNUBBED)) and snubber capacitance are recommended for all offline application and connected, (CSNUBBER) is below, and the resultant waveforms are as shown in Figure 3. The TVS device needs a reverse shown in Figure 4. breakdown voltage greater than (VOUT + VF) • NPS where VOUT is the output voltage of the flyback converter, VF is CPAR = CSNUBBER 2 the secondary diode forward voltage, and N   PS is the turns tPERIOD(SNUBBED) – 1 ratio. An RCD clamp can be used in place of the TVS clamp.    tPERIOD  VSUPPLY VSUPPLY 2 LPAR = tPERIOD CPAR • 4π2 RSNUBBER = LPAR CPAR GATE GATE 90 80 70 3798 F03
Figure 3. TVS & RCD Switch Voltage Clamps
60 (V) 50 40 In addition to clamping the spike, in some designs where V DRAIN 30 short circuit protection is desired, it will be necessary to NO SNUBBER 20 WITH SNUBBER decrease the amount of ringing by using an RC snubber. CAPACITOR WITH RESISTOR Leakage inductance ringing is at its worst during a short 10 AND CAPACITOR circuit condition, and can keep the converter from cycling 0 0 0.05 0.10 0.15 0.20 0.25 0.30 on and off by peak charging the bias capacitor. On/off TIME (µs) cycling is desired to keep power dissipation down in the 3798 F04 output diode. Alternatively, a heat sink can be used to
Figure 4. Observed Waveforms at MOSFET Drain when
manage diode temperature.
Iteratively Implementing an RC Snubber
The recommended approach for designing an RC snubber is to measure the period of the ringing at the MOSFET Note that energy absorbed by a snubber will be converted drain when the MOSFET turns off without the snubber to heat and will not be delivered to the load. In high volt- and then add capacitance—starting with something in age or high current applications, the snubber may need to the range of 100pF—until the period of the ringing is 1.5 be sized for thermal dissipation. To determine the power to 2 times longer. The change in period will determine dissipated in the snubber resistor from capacitive losses, the value of the parasitic capacitance, from which the measure the drain voltage immediately before the MOS- parasitic inductance can be determined from the initial FET turns on and use the following equation relating that 3798fa 13 Document Outline Description Features Applications Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Typical Application Related Parts