Discrete Design of a Low-Cost Isolated 3.3V to 5V DC/DC Converter

Isolated 3.3V to 5V converters are often used in long-distance data transmission networks, where the bus node controller operates from a 3.3V power supply to save power, while the bus voltage is 5V to ensure signal integrity and provide high drive capability during long-distance transmission. Although isolated DC/DC converter components for 3.3V to 5V conversion are already available on the market, integrated 3.3V to 5V converters are still difficult to find. Even if found, these specific converters (especially those with regulated outputs) usually have long product delivery times, are relatively expensive, and generally have certain isolation voltage limitations.

If the application requires isolation voltages greater than 2 kV, converter efficiency greater than 60%, or the proven availability of standard components, then discrete designs are a low-cost alternative to integrated components. The downside to discrete DC/DC converter design is that it requires a lot of work—selecting a stable oscillator structure and break-before-make circuit, choosing MOSFETs that can be effectively driven by standard logic gates, and properly performing temperature and long-term reliability testing. All of these efforts cost time and money. Therefore, before rushing into such a plan, designers should consider the following: Integrated components are usually tested over temperature and have other industrial qualifications. Not only are these components the most reliable solution, they also have a faster time to market.

Astable output converters typically start at $4.50 to $5.00 in 1,000-unit quantities, while regulated output converters are usually twice that price, around $10.00 or more. Therefore, it makes sense to buy a converter with an astable output and either buffer the output with a buck capacitor or feed it into a low-cost, low-dropout regulator (LDO), such as TI’s TPS76650.

The discrete DC/DC converter design shown in Figure 1 uses only a few off-the-shelf standard components such as logic ICs and MOSFETs for the transformer driver and an LDO for output voltage regulation. The circuit was prototyped using many through-hole components, making it larger than an integrated package, but board space was greatly reduced by using TI's Little Logic™ devices.

The main benefit of this design is a smaller bill of materials (BOM) and the freedom to choose the isolation transformer for isolation voltages ranging from 1 to 6kV. Our goal is to provide a low-cost alternative to fully integrated DC/DC converters and stand-alone transformer drivers by making the transformer driver stage a stable output.

Figure 1. Isolated 3.3V to 5V push-pull converter

How it works

Low-cost, isolated DC/DC converters are generally of the push-pull driver type. The operating principle is very simple. A square wave oscillator with a push-pull output stage drives a center-tapped transformer, whose output is rectified and can be used in a regulated or unregulated DC form. An important functional requirement is that the square wave must have a 50% duty cycle to ensure symmetrical magnetization of the transformer core. Another requirement is that the product of the magnetizing voltage (E) and the magnetizing time (T), called the ET product in Vμs, must not exceed the typical ET product of the transformer specified by its manufacturer. We must also use a break-before-make circuit installed close to the oscillator to prevent the two transformer core legs of the push-pull output stage from conducting at the same time and causing circuit failure.

Discrete design

The well-known three-inverting gate oscillator consisting of U1a, U2a, and U2b was chosen because it is relatively stable in terms of power supply fluctuations. Its normal frequency is set to 330kHz by a 100-pF ceramic capacitor (COSC) and two 10-kΩ resistors (ROSC1 and ROSC2). In the 3.0-V to 3.6-V power supply voltage fluctuation range, the oscillator has a duty cycle close to 50% and a maximum frequency fluctuation of less than ±1.5%. Figure 2 shows the waveforms at the summing point of ROSC1 and ROSC2 (TP1) and the oscillator output (TP2). All voltages are measured with reference to the circuit reference voltage.

Figure 2 Oscillator waveforms at TP1 and TP2

The Schmitt trigger circuit NAND gates (U1c, U1d) implement the break-before-make function to avoid overlapping of the MOSFET conduction phase. The other two NAND gates (U2c, U2d) are configured as inverting buffers to generate the correct signal polarity necessary to drive the N-channel MOSFETs (Q1, Q2). Figure 3 shows the complete break-before-make action. To accommodate the limited drive capability of standard logic gates, MOSFETs were selected due to their lower total charge and fast response time.

Figure 3 Break-before-make waveform

The isolation transformer (T1) has a 2:1 secondary-to-primary turns ratio, 0.9 mH primary inductance, and a guaranteed isolation voltage of 3 kV. Figure 4 shows the transformer input and output waveforms.

Figure 4 Transformer waveform

Both diodes (D1, D2) are fast Schottky rectifiers that provide a low forward voltage at full load current (VFW < 0.4 V at 200 mA) while performing full-wave rectification. It is possible to obtain the output voltage directly from the buck capacitor (Cb3) after these diodes. In this case, the output is not regulated, but with maximum efficiency of the DC/DC converter. However, the designer must ensure that the maximum supply voltage of the affected circuit is not exceeded, which is more likely to occur at low load or open circuit conditions. If the unregulated output voltage at minimum load is too high, a linear regulator must be used after the full-wave rectifier to provide a regulated output supply voltage.

The main benefit of a linear regulator is low ripple output. Other benefits include short circuit protection and over-temperature shutdown. However, the main disadvantage is very low efficiency.

Figure 5 shows the ripple of the circuit in Figure 1 at an output voltage of 4.93 V, while Figure 6 compares the efficiency of the circuit with an integrated DC/DC component with a regulated output.

Figure 5 Output ripple when VOUT=4.93V

Figure 6 Efficiency comparison

The table below provides the BOM for a discrete DCDC converter. Note that the bypass capacitor value is larger than the 10 nF commonly used in some low-speed applications. This is due to the high dynamic loads of high-speed CMOS technologies (such as AHC, AC, and LVC, etc.), so the bypass capacitor value must be 0.1 μF or higher to ensure normal operation. This is especially important for the inverting buffer driving the MOSFET, where the bypass capacitor value is 0.68 μF.

in conclusion

When there are no board space constraints, a discrete design of an isolated 3.3-V to 5-V DC/DC converter with a regulated output can be a realistic, low-cost alternative to regulated output integrated DC/DC components. The main benefit of a discrete design is the freedom to choose the isolation transformer to meet a variety of isolation voltage requirements.