What are HVICs?
High-voltage integrated circuits (HVICs) translate low-voltage control signals to levels that are suitable for driving power switches in high-voltage applications. HVICs also can translate signals from high voltage levels to lower voltage levels.
A basic HVIC might provide simple-up or shift-down capability, while a more advanced one might provide half- or full-bridge drive capabilities. Yet another might be specially designed for Class D audio amplification, or it might provide multiphase outputs for motor control.
Because of their monolithic construction, various enhancements can be integrated into HVICs. These include advanced protection for the external switches through voltage and/or current sensing and a complete analog or digital control circuit to provide auxiliary functions inside or outside the HVIC.
Where are HVICs used?
Basic HVICs find uses in many power-conversion applications where signals from a low-voltage controller are required to turn switches between high-voltage power rails on and off to deliver power to a load. Figure 1 shows a class-D audio application for an HVIC, but the basic circuit topology would be the same in many applications.
Application-specific HVICs offer all the functional blocks required to realize a full application. Specific applications include appliance motor controllers, off-line power supplies, uninterruptible power supplies, battery chargers, fluorescent lighting ballasts, and plasma display panels.
How may the additional functions in an HVIC be used?
Specific HVIC types can range from simple gate drivers with little or no control to sophisticated drivers that include control and/or sensing functions for special applications. For example, motor controllers can use built-in current sensing functions in special HVICs to relay the condition of the load current to a microcontroller to monitor and protect the motor in case of adverse conditions. Figure 2 shows such an HVIC application where the current in the high-side switch is sensed and transferred to the low side in the form of a pulse-width modulated (PWM) signal.
Another popular application for HVICs is in lighting. There are HVICs for driving lamps ranging from simple fluorescents to more sophisticated high-intensity discharge (HID) lamps. HVICs are also increasingly being used in applications such as Class D audio amplifiers, appliances using induction heating, and piezoelectric fuel injectors in high-pressure diesel engines.
Are HVICs the only approach to high-voltage level shifting?
There are alternative ways to level-shift signals from one reference to another, like pulse transformers and other magnetically isolated level shifters, optocouplers, and capacitive-coupled level-translating drivers. Generally, though, HVICs are more attractive than these alternatives.
For example, HVICs have lower losses than optocouplers. This is primarily because they don’t require secondary-side power supplies. (Secondary power can be derived from the primary side using bootstrapping techniques requiring simply a diode and capacitor.) Another advantage is that they can accommodate any practical output drive size. Then, there are HVICs designed for output currents in excess of 4 A (peak) for driving large switches.
Other advantages over optocouplers include high-frequency operation and short input-to-output delays. HVICs can function up to 1 MHz, and delays are less than 50 to 60 ns. Compared to pulse transformers, HVICs offer larger duty cycle ranges. More broadly, as monolithic circuits, HVICs can provide functionality beyond level shifting at little or no extra cost and with minimal external circuitry. For instance, undervoltage lockout inhibits the outputs in case there’s a power-supply brownout.
Are there disadvantages to HVICs?
Only one: HVICs do not provide galvanic isolation, whereas optocouplers and some of the other solutions do. However, galvanic isolation can be achieved in HVICs by moving the isolation barrier to the input signals.
How do I choose an HVIC for an application?
First, check whether there is an HVIC designed specifically for your application. Application-specific HVICs offer tremendous cost savings over designing a controller from scratch. If you cannot find a specific HVIC, start by determining the maximum voltage of your application. HVIC voltages can range from 100 to 1200 V. Next, decide on functionality. Do you need a simple driver or a bit more, like a half-bridge or three-phase driver?
Consider the input, output, and switching frequency. What will drive the HVIC? HVICs can accommodate 3.3-, 5-, and 15-V input logic, with most having a 20-V maximum at the inputs. Other questions include whether the HVIC will be working at 10 to 100 kHz or 300 to 500 kHz. Is input/output delay important in this application? Is delay matching of the high-side and low-side sections important? What are the output drive current requirements? Are you driving small switches or large ones? Will you be paralleling some of them?
HVIC output drives can range from few hundred milliamps to well over 4 A. Which one do you need to fulfill your timing requirements? Finally, consider which protection features your application requires. Do you need a "shut-down" pin? Do you need current sensing?
What must I watch for when designing with HVICs?
Watch out for fast dv/dts on the floating well. While most HVICs can withstand well over 50-V/ns transitions, higher transitions might cause false triggering in the high side. Keep transition dv/dts below the maximum recommended limit. Also, the high-side well needs to always be biased at or above the lowest potential in the circuit. Due to the HVIC’s internal parasitic components, damage could result if the well is pushed below the recommended level for an appreciable time. And as the natural environment for HVICs is noisy, the layout of switches and the HVIC is critical for the proper operation of the system. Keep switches and the drivers in close proximity, and use short traces to minimize inductance and avoid ringing on the gates.