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need for an optocoupler, secondary-side reference voltage and extra third winding off the power transformer, all while maintaining isolation between the pri- mary- and secondary-side with only one part required to cross the isolation barrier. This device employs a primary-side sensing scheme that is capable of detect- ing the output voltage through the flyback primary-side switching node waveform. During the switch off-period, the output diode delivers the current to the output, and the output voltage is reflected to the primary-side of the flyback transformer.
The magnitude of the switch node volt- age is the summation of the input voltage and reflected output voltage, which the LT3748 is able to reconstruct. This output voltage feedback technique results in more than ±5 percent total regulation over the full line voltage input, temperature range and load range. Figure 1 shows a flyback converter schematic using this device. The controller accepts an input volt- age from 5V to 100V that can be applied directly to the IC. It can reliably operate a high input voltage due to the high voltage onboard LDO and housed in an MSOP- 16 with 4 pins removed for high voltage spacing. In addition, its onboard gate driver powers an external N-channel power MOSFET, enabling it to deliver up to about 50W with its maximum output power dependant upon external compo- nent selection, the input voltage range and output voltage. In addition, the LT3748's utilisation of boundary mode operation further sim- plifies system design and reduces the overall converter size and footprint. The flyback converter turns on its internal switch immediately after the secondary side current reduces to zero and turns off when the switch current reaches the pre-defined current limit. Thus, it always operates at the transition of con- tinuous conduction mode (CCM) and discontinuous conduction mode (DCM), commonly referred to as boundary mode or critical conduction mode.
Primary-side voltage sensing Output voltage sensing for an isolated converter normally requires an optocou- pler, secondary-side reference voltage and opto driver. An optocoupler trans- mits the output voltage feedback signal through the optical link, while main- taining the isolation barrier. However, an optocoupler transfer ratio changes with temperature and aging, degrading its accuracy. This device eliminates the need for an opto- coupler, reference voltage and opto driver by sensing the output voltage on the primary-side of the transformer. The output voltage is accurately measured at the primary-side switching node waveform during the off time of the power transistor as shown in Figure
Electronics JUNE 2012
Figure 1: LT3748 Flyback Converter with Primary-Side Output Voltage Sensing
A tolerance of ±5 percent in turns ratio from transformer to transformer could result in a variation of more than ±5 percent in output voltage. Fortunately, most magnetic component manufactures are capable of guarantee- ing a turns ratio tolerance of ±1 percent or better. Linear Technology has worked with leading magnetic component manufac- tures to produce pre-designed flyback transformers for use with the LT3748. Table 1 shows an abbreviated list of recommended off-the-shelf transform- ers from both Wurth Electronics and Pulse Engineering.
A complete listing can be found in
the LT3748 data sheet. These transform- ers typically withstand a 1,500V AC breakdown voltage for one minute from the primary to the secondary. Higher breakdown voltage and custom trans- formers can also be utilised. The LT3748 can be modelled using
Figure 2: Typical Switch Node Waveform
2 where N is the turns ratio of the trans- former, VIN
is the input voltage and VC and RREF is
the maximum clamped
voltage.The converter sets the output voltage with the selection of RFB
, and the trans- former's turn ratio.
The load regulation is much improved in boundary mode operation because the reflected output voltage always samples at the diode current zero-crossing. This converter typically provides ±3 percent load regulation.
Transformer selection and design considerations
The transformer specification and design is probably the most critical part of successfully applying the LT3748. In addition to the usual list of caveats deal- ing with high frequency isolated power supply transformer design of having a low leakage inductance and close cou- pling, the transformer turns ratio must be tightly controlled. Since the voltage on secondary side of the transformer is inferred by the voltage sampled on the primary-side, the turns ratio must be tightly controlled to ensure a consistent output voltage.
Figure 3: LT3748 Application 30W Circuit (Size: 38mm by 19mm by 9.5mm)
LTspice (free download) with any of the transformers listed in Table 1. The simu- lation circuit includes information on how the circuit starts up, its reaction to a load step and voltagewaveforms at vari- ous points within the circuit. It's easy to make design changes and see the impact this has to its circuit performance. A demonstration board using this converter is shown in Figure 3. This circuit accepts an input voltage ranging from 22V to 75V and produces an isolated 12V output at up to 2.5A. Although the design of an isolated fly- back converter is not a simple task, there is now an alternative to using modules or a complex discrete implementation. An LT3748 based circuit simplifies the design by eliminating the need for an optocoupler, secondary-side reference voltage and opto driver. It maintains pri- mary to secondary isolation with only one part crossing the isolation barrier. Readily available off-the-shelf trans- formers prevent the need for a custom transformer. The LT3748 operates from a 5V to 100V input voltage range and has the ability to deliver up to 50 Watts of continuous output power, making it suitable for a wide range of security IP camera applications. Linear Technology
www.linear.com Enter 204
Table 1: Off-the-Shelf Transformers for Use with the LT3748 Target
Application Supplier
20V to 75V 12V/1A Pulse 20V to 75V 5V/2A Pulse 10V to 40V 10V/2.5A Pulse
18 x 14 x 13 2.7:1 18 x 14 x 13 3.67:1 29 x 22 x 11.5 2:1
30V - 75V 12V/1A Wurth/Midcom 18 x 14 x 13 3:1 30V - 75V 5V/3A Wurth/Midcom 18 x 14 x 13 4:1 10V - 40V 5V/3A Wurth/Midcom 18 x 14 x 13 3:1 20V - 75V 12V/2.5A Wurth/Midcom 29 x 23 x 11.5 3:1
Size Turns Ratio Part (WxLxH) (mm) NP:NS
Number
PA2367NL PA1260NL PA3177NL 750311424 750311423 750311564 750311607
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