SLUU085 – November 2001
6
UCC3580EVM
3.2
Added Features
Flyback converters require a voltage clamp in order to limit the main switch voltage at turnoff. The effect is due
to energy that is stored in the primary leakage inductance of the power transformer. Older hard-switched
technology captures the excess leakage inductance energy at turnoff and merely dissipates it in a resistor. In
contrast, the active clamp uses the leakage inductance energy to facilitate zero voltage transitions. Efficient ZVS
transitions occur at turnon of both the main MOSFET and the auxiliary MOSFET.
Active-clamp technology is made possible here with the UCC3580 single ended active clamp/reset PWM
controller. Essentially, this controller is a voltage mode PWM controller with complementary outputs and
programmable delay between output transitions. The controller also includes over-current protection,
volt-seconds protection and a soft-start feature. The programmable delay is set to be long enough for the
leakage inductance energy to forward bias the power MOSFETs before their respective turnon, thus achieving
zero voltage switching.
The schematic for the active clamp configuration is shown in Figure 2 and its parts list is in Table 2. The
schematic for the hard-switched configuration is shown in Figure 3. Both configurations operate with the same
input voltage (110 VAC + 10%) and produce the same output voltage (12 V) and current (10 A). Heat sink HS1
is selected to accommodate the hard-switched configuration and it is thus over-sized for active clamp operation.
The thermal resistance of HS1 is 6.5
°
C/W, which is required for hard-switched operation. In contrast, heat sink
HS1 only needs a thermal resistance of 10
°
C/W for active-clamp operation, which was verified in the laboratory.
Thus transistor QM operates more efficiently, and it requires a smaller heat sink in the active-clamp
configuration. Also, notice that the active-clamp configuration does not require secondary rectifier snubber
components (R31 and C30). These features are the result of efficient active clamp technology.
For more information, pin descriptions and specifications for the UCC3580 single ended active clamp/reset
PWM, please refer to the UCC3580 data sheet.
3.3
Re-Configuration Instructions
WARNING:
Danger: Remove power from the UCC3580EVM before removing or installing
components.
These instructions describe how to reconfigure the UCC3580EVM from the active clamp configuration to the
hard switched configuration. This section requires components, de-soldering, soldering, and assembly
equipment and skills. The components are listed in Table 4.
Begin with an UCC3580EVM in the active-clamp configuration (original configuration). Refer to Table 3 and
Table 4 for the components to remove and add, respectively in order for the UCC3580EVM to match the
schematic in Figure 3. The additional components in Table 4 are supplied in the kit that is included with the
UCC3580EVM.
De-solder the components listed in Table 3. Solder the components listed in Table 4.
Conversion is now complete and comparisons can be made using the test setup in Figure 1 and the test points
in Table 1.