Gate drive transformers vs. high-side and low-side drivers: Where is power supply design headed?

    In a typical closed-loop power electronics system, the gate driver is the critical interface between the control system (typically low voltage, such as 12V) and the main power stage (typically high voltage, such as 400V DC). The gate driver's purpose is to convert the input low-voltage control pulse signal to the power transistor (MOSFET, IGBT) in a clean, robust, and timely manner.

    In this article, I will introduce two methods of driving high-voltage transistors: gate drive transformers and high-voltage driver ICs, and explain the advantages and disadvantages of each method.

    The key specifications that define gate driver performance are:

    Static characteristics: functional voltage (VCC/DD, bootstrap function), peak source/sink current, and UVLO.

    Dynamic characteristics: propagation delay, delay matching, pulse width distortion, common-mode transient immunity (dv/dt), and rise/fall times.

    We also need to consider safety standards and regulatory compliance—protecting operators from hazardous voltages and currents exceeding 42.4V pk AC or 60V DC. For example, in a mobile phone charger, the low-voltage DC output is isolated from the universal AC input (85-265V AC). In this case, double or reinforced insulation is required to eliminate the need for a grounded metal enclosure and a grounded power plug. Table 1 shows the test voltage requirements for solid insulation in main circuits with Category II overvoltages up to 300V (IEC 61010-1 ed. 3.0).

Table 1: Test voltages for solid insulation in main circuits of overvoltage category II

    Figure 1 shows a simplified circuit diagram with the controller located on the secondary side (secondary-side control). The primary power stage isolation is based on a traditional power transformer. Two main types of gate drivers can be used to transmit the gate drive signals and isolate the feedback control between the secondary and primary-side gate drivers:

    A gate drive transformer with isolation achieved through magnetic coupling (see Figure 2[a]).

    High-side and low-side gate drivers with signal isolator interfaces (see Figure 2[b]). The signal isolator interfaces can be optocouplers (optical coupling) or digital isolators (magnetic or capacitive coupling).

Figure 1: Simplified circuit diagram with secondary-side control

    The gate drive transformer can provide the logic gate drive signals and the required peak current/power capability required by the gate driver.

    The high-side and low-side gate drivers use a signal isolator interface to provide the required isolation and a gate driver IC to provide sufficient gate drive power/current capability.

Figure 2: Simplified circuit diagram (a) Gate drive transformer (b) High-side and low-side drivers + isolator

    Table 2 lists the key components required for each implementation. The gate drive transformer uses the UCC27324 as a low-side driver with two channels capable of ±4A Ipk, driving a gate drive transformer from Coilcraft and the GA3550 with reinforced insulation. The high-side and low-side gate drivers plus isolators use the ISO7520C dual-channel digital isolator with reinforced isolation, the UCC27714 as the high-side and low-side gate drivers, and the Vishay MURS360 as the bootstrap diode.

    The UCC2732x and UCC3732x families of high-speed dual MOSFET drivers offer 4A source and 4A sink peak currents, effectively driving MOSFETs in the most demanding Miller Plateau region. A unique combination of BiPolar and MOSFET paralleling in the output stage also allows for efficient current sourcing and sinking at low supply voltages. Three standard logic options are available—dual inverting and dual non-inverting drivers, as well as single inverting and single non-inverting drivers. Input thresholds are TTL- and CMOS-based, independent of supply voltage, with wide input hysteresis for excellent noise immunity. The UCC2732x and UCC3732x families are available in standard SOIC-8 (D) or PDIP-8 (P) packages, as well as a thermally enhanced 8-pin PowerPAD MSOP package (DGN), which significantly reduces thermal resistance for improved long-term reliability.

    The UCC27714 is a 600V high-side/low-side gate driver with a 4A source/sink current capability, designed for driving power metal-oxide semiconductor field-effect transistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs). The device includes a ground-referenced channel (LO) and a floating channel (HO) designed for bootstrap power supply operation. The device offers excellent stability and noise immunity, maintaining normal logic operation with negative voltages up to –8VDC on the HS pin (at VDD = 12V).

    Take a look at the total minimum PCB area required in Table 2: Type II (high-side and low-side drivers plus isolators) takes up only 215 mm², saving over 50% PCB space compared to Type I. And the savings are even more significant when the awkward height of the reinforced insulation gate drive transformer is taken into account.

    Furthermore, this calculation only considers the primary components. The savings of Type II over Type I increase when signal conditioning circuitry is considered.