LT3752, LT3752-1 and LT3753 are high-performance active clamp forward controllers with high integration, which can minimize the number of external components, solution size and cost reduction. LT3752 and LT3753 are used for up to 100V input, while LT3752-1 is used for applications with input voltage higher than 100V. Therefore, LT3752-1 is suitable for HV automotive batteries and off-line isolated power supply, as well as industrial, automotive and military systems. All devices can be used to provide compact, versatile and efficient solutions for single-chip IC with output power up to 400W. The output of the cascade stack converter can also support greater power. See Table 1 to compare the functions of these devices. Figure 1 shows a complete 150W forward converter, which does not require an optical coupler due to the accurate programmable volt-second clamping of LT3752. For forward converters operating in continuous on mode, the output voltage is VOUT = VIN. N. D, where VIN is the input voltage, N is the turn ratio from secondary side to main side, and D is the duty ratio. Duty cycle clamp circuits on LT3752, LT3752-1 and LT3753 OUT pins track VIN negatively to maintain a constant VOUT in the input voltage range. In active volt-second clamping circuit, the accuracy of Vour depends largely on the accuracy of volt-second clamping part. Similar voltage clamp solutions use an external RC network connected to the system input to trigger the jump threshold of the internal comparator.
The accuracy of this RC method is limited by the errors of external capacitors, the mismatch between the RC time constants of devices and devices and IC switching cycles, the threshold errors of internal comparators, and the nonlinearity of charging at low input voltages. To ensure accurate device-to-device adjustment, LT3752, LT3752-1 and LT3753 provide fine-tuned timing capacitors and comparator thresholds. Figure 2 shows the curve of VOUT varying with load current at various input voltages. If the duty ratio clamp resistor is open, the device immediately stops switching, thus preventing the device from running without volt-second clamp. LT3752/LT3752-1 includes an internal constant frequency flyback controller to generate internal processing power. The internal processing power supply can efficiently provide bias for the main and secondary side IC, thus eliminating the need to generate bias power from the auxiliary windings of the main forward transformer, which significantly reduces the complexity, cost and size of the transformer. The internal processing power supply can be used to overdrive the INTVcc pins to obtain power beyond the device limits, improve efficiency, provide additional driving current and optimize the INTVcc level. Before the switching of the main forward converter starts, thermostatic element the internal processing power supply can also provide bias to any side IC. In this way, no external start-up circuit is needed for the secondary side. Accurate LT3752/LT3752-1 under-voltage locking (UVLO) function can be used to achieve power sequencing or start over-current protection. Simply add a resistor voltage divider between the VIN power supply and the UVLO pin. The UVLO pin can adjust the input hysteresis, allowing the IC to resist input power drop before performing soft stop.
During soft stop, converter turn back switch frequency, volt second clamp and comp pin voltage will continue to switch. The UVLO pins of LT3752, LT3752-1 and LT3753 all have a miniature power shutdown threshold of about 400 mV, and the static current of VTN decreases to 40 mu A or lower. Soft start function can be realized by adding capacitors to soft start pins (SS1 and SS2). Soft start reduces peak input current and prevents output voltage from overshooting when starting or recovering from failure.
SS1/2 pin reduces surge current by reducing current limitation and switching frequency, thus allowing output capacitors to gradually charge to final power. In contrast to soft startup, LT3752/LT3752-1 and LT3753 can gradually discharge SS1 pins (soft stop) during shutdown. Figure 3 shows the downtime waveform of the converter in Figure 5. If there is no soft stop, the feedback of self-driving synchronous rectifier will transmit the capacitor energy to the main side, which may lead to downtime oscillation and damage the main side components. Figure 4 shows the downtime waveform of soft stop.
When the converter turns back the switching frequency, volt-second clamp and COMP pin voltage, it continues to switch, thus achieving a clean shutdown. LT3752/LT3752-1 and LT3753 adopt current mode control architecture to provide larger power bandwidth and better voltage and load transient response than voltage mode controller.
Compared with the voltage mode control architecture, current mode control requires fewer compensation components, which makes it easier to compensate for a variety of working conditions. For continuous mode operation and duty cycle operation above 50%, the required slope compensation can be set by a single resistor.
LT3752/LT3752-1 and LT3753 include programmable functions that allow designers to optimize for specific applications. For example, programmable delays between different gate signals can be used to prevent cross-conduction and optimize efficiency. Each delay can be set by a single resistor. The programmable on-current peak blanking (adaptive front blanking and programmable extended blanking) of the main MOSFET greatly improves the noise resistance of the converter. Noise may occur in current detection resistors connected to the MOSFET source as the gate rises (sometimes later). The noise may cause the detection comparator to jump and lead to switch off ahead of time. One solution to this problem is to use large-scale RC filters to prevent miss jumps, but with programmable on-peak blanking, no additional RC filters are needed. The operating frequency can be set by a single resistor between the RT pin and the ground in the range of 100-500 kHz, or synchronized to an external clock through the SYNC pin. The adjustable operating frequency allows the device to be set outside certain frequency bands to adapt to spectrum noise sensitive applications. Fig. 5 shows a 5V, 20A output converter that accepts 36 to 72V input. The active reset circuit consists of a small P-channel MOSFET M2 and a reset capacitor.
MOSFET M2 is used to connect reset capacitors across the main side windings of transformer Tl during reset when the ML MOSFET is turned off. Voltage at both ends of reset capacitor is automatically adjusted with duty cycle to provide complete reset of transformer under all operating conditions. In addition, the active reset circuit converts the reset voltage waveform into a square wave, which is suitable for driving the secondary synchronous MOSFET rectifier M4. These MOSFETs are located at the secondary side and are driven by the secondary winding voltage. Figure 6 shows the efficiency of the converter. Figure 7 shows an 18V-72V input and 12V/12.5A output forward converter.
LT8311 is used on the side of the forward converter to provide synchronous MOSFET control and output voltage feedback through an optical coupler. A pulse transformer is needed (see T3 in Figure 7) to enable LT8311 to receive synchronous control signals from the main side IC. These control signals are converted into digital signals (high or low) by LT8311 to turn on/off clamp and forward MOSFET. Figure 8 shows the efficiency of the converter. For applications with high input voltage, the voltage rating of available P-channel MOSFETs may not be high enough to be used as active clamp switches in low-voltage side active clamp topologies. An N-channel method with active clamping topology on high voltage side should be adopted.
This topology requires a high-voltage side gate driver or a grid transformer to drive N-channel MOSFET and to connect the active clamp capacitor through a switch.