Gate Drive Transformers Simplify Design of Multiple-Output Isolated DC-DC Converters

　　Typically, the biggest obstacle encountered when designing an isolated DC-DC converter is transformer design, which often discourages designers from doing so and leads to other simpler design tasks. By utilizing the characteristics of commercially available gate drive transformers, four separate isolated DC outputs can be obtained. In fact, for low-power DC-DC power conversion, the gate drive transformer is an ideal choice because this transformer has been optimized for a large voltage and time product (ET or volt-microsecond product) and low leakage inductance.

　　A core with high permeability and low losses at high switching frequencies (FSWX) supports typical 10V~15V primary voltages and a typical on-time of 500ns~5μs at 100kHz~500kHz switching frequencies. This voltage and time range is exactly what the DC-DC converter design requires. At the same time, a core geometry and winding structure have been selected for low leakage inductance to reduce rise and fall times while having low ringing. Finally, the wire gauge used is sufficient for the DC-DC converter to handle winding currents of tens of mA without excessive copper wire losses.

　　The P0585 gate-drive transformer from Pulse Electronics has five windings, each with the same number of turns (Reference 1). One winding uses triple-insulated wire (TIW), and the other four use standard winding wire. The TIW winding is used as the primary drive, which achieves a nominal primary-secondary breakdown voltage of 3 kV RMS. The rated breakdown voltage between the four secondary windings is not specified, but this type of wire insulation is often used in offline power supply situations, where the voltage between the windings can be as high as 400 V.

　　Isolated power outputs offer a lot of flexibility. Using this approach makes it easier to break ground loops, power remote circuits at different ground potentials, and simplify the selection of positive and negative output voltage polarity. The figure below shows the four secondary windings of this transformer, which produce four independent equal voltage outputs. But these four secondary windings can have a variety of series/parallel combinations, resulting in a large number of output voltage/current combinations.

　　The MAX13256H-bridge transformer driver (IC1) from Maxim Semiconductor is best suited for this application. It contains all the functions required for a standalone transformer-isolated DC-DC converter. Its built-in FETs can withstand 36V and are configured as two independent push-pull outputs to drive the primary of the transformer with a precise 50% duty cycle, avoiding core saturation. The driver also includes an adjustable, robust internal current limit function, which provides short-circuit protection for the output and can recover gracefully after the fault is removed. The driver also includes an undervoltage lockout (UVLO) function to prevent switching activity when the input voltage is too low.

　　The addition of a Linear Technology LTC6900 clock source (IC2) is used to precisely adjust the switching frequency. The MAX13256 itself has an internal clock, but most users may prefer to set their own switching frequency for overall system compatibility or EMI reasons. The MAX13256 supports the use of an external TTL-level clock, and its UVLO feature ensures that IC2 is powered up and running before IC1's VIN pole rises to the turn-on threshold. The value of RSET determines the output frequency of IC2, which is set to twice the desired IC1 switching frequency.

　　The table above shows the measurement results for input voltages of 10V, 12V, and 15V at switching frequencies of 100kHz and 500kHz. Due to the high switching frequency, low leakage inductance, and Schottky bridge rectifier, the output voltage ripple is very small, less than 20mV peak-to-peak, even with low-value (1μF) surface-mount ceramic output capacitors. The table also shows the efficiency and the relationship between the output voltage and load current caused by the unregulated output. If lower noise or strictly regulated DC output is required, a linear regulator can be used to regulate the output voltage.

　　From these measurements, it can be seen that full load is the load that produces 500mA peak initial current. This is the minimum current-limit threshold of the MAX13256 when RLIM is 1kΩ. Some designers may want to operate below these empirical full-load current levels to gain more margin to prevent false high-current triggering. The increase in light-load output voltage can be seen at higher switching frequencies because this is a design that uses less snubber for simplicity and efficiency. As the switching frequency increases, more leakage inductance energy is generated, which is transferred to the secondary winding, increasing the measured output voltage.

　　The following is a brief description of how to verify that the transformer is operating within the data sheet specifications. The P0585 transformer has a maximum ET product of 95Vμsec. This calculation is the product of the maximum voltage applied to the primary winding and the maximum time that voltage is sustained (the on-time). Since the MAX13256 drives the transformer primary at a precise 50% duty cycle, the maximum ET product will occur at an input voltage of 15V. At the minimum switching frequency of 100kHz in this design, the maximum on-time is 5μs, so the maximum ET product is 75Vμsec, which is within the data sheet specification.

　　The peak flux density is specified as 2100G. In calculating the peak flux, Equations 2A and 2B in the data sheet are based on VIN and switching frequency. Again, the peak flux density occurs when VIN is 15V and the switching frequency is 100kHz. Note that in Equation 2A, “DON” is 50% duty cycle or 0.5, not time in microseconds. Under these conditions, the calculated peak flux density is 1512 Gauss, which meets the specifications in the data sheet.

　　Using the formula from the transformer data sheet, the core losses are calculated to be 0.468W at 100kHz and 0.117W at 500kHz, the latter being lower due to the lower ET product.

　　Using the formula from the transformer data sheet, the copper losses are calculated to be 93.75mW. This simplified formula for copper losses is based on the I2R losses of the windings and does not take into account the skin or proximity effects of the windings. Therefore, these simplified results are independent of frequency and are based on ±500mA peak currents in the primary winding and ±125mA peak currents in each of the four secondary windings.

　　Using the temperature rise formula from the transformer data sheet and the total losses calculated above (561.75mW at 100kHz), the expected transformer temperature rise is 37.2°C.

　　This design uses the P0585 gate drive transformer, but you can use other (smaller) commercially available gate drive transformers, especially if you need less voltage output, and less current. Just make sure you check the maximum Vμsec specification of the transformer as described in this design.