Integrating High Voltage Modules into Critical, Long-Term Applications
High voltage design-in process presents numerous challenges in various applications. These challenges have presented themselves through observations in the field and conversations with design engineers over the years.
Some of these challenges are listed below:
- Defining the high voltage module requirements
- Accuracy of high voltage power converter
- Design of the circuitry around high voltage in the application
- Ease of handling and integration in the end application
Defining the high voltage module requirements
Defining requirements is critical in the design process. Below is the list of a few things that should be considered when looking for high voltage modules.
- Input and output conditions: Understanding the input available to the high voltage module and loading conditions in the application is the first step towards getting the right solution. The input voltage along with its accuracy is important and helps define the required line regulation and protection requirements from the module. The load in most of the applications that one comes across is a hybrid of resistive and capacitive elements. So, understanding the load and loading conditions is critical in defining the voltage and current requirement for the high voltage supply.
- Voltage and current requirement: Based on what the high voltage needs to do, voltage and current along with polarity need to be defined. This mostly depends on the load specifications. For example, a Photomultiplier Tube may require 1200VDC with micro-Amps of current.
- Control and monitor signals: As most of the application in today’s world are controlled by digital circuitry, understanding the signals that you have available in your application to control and monitor the high voltage module is critical for easier integration.
- Environmental conditions: Depending on the application, there may be specific environmental conditions like operating temperature, humidity etc. that need to be considered. Pay attention to where the application is going and where in the application the high voltage module resides.
- Size constraints: This corresponds to the space availability for the high voltage supply and overall application. In the small desktop and handheld applications size becomes very critical. Even in applications that require more space like semiconductor fabrication, inspection tools, and analytical instruments the focus has been to make them as small as possible to put more functionality into the same space or make space available for more new generation instruments.
- Agency approvals: This becomes critical where the end application is going into an industry segment that has certain regulatory approval requirement. For Example, UL/IEC/EN 61010 for the analytical instrument market.
Accuracy of high voltage power converter
Accuracy of the high voltage power converter is affected by other variables which may include variation in the input voltage, loading conditions, operating temperature, and other environmental factors. Below is the list of the specifications that should be considered while defining the high voltage accuracy requirement for the module.
- Output voltage tolerance: Variation from the specified voltage.
- Setpoint accuracy: Ability of the supply to achieve the set point using the control voltage.
- Line and load regulation: Ability of the supply to hold the output voltage within the regulation specification based on the variation in the input voltage and the load.
- Ripple and Noise: The residual AC signal on top of the DC high voltage supply.
- Linearity of the control. Transfer function of the output voltage with respect to the control voltage.
- Temperature coefficient: The change in output voltage with respect to the change in temperature per degree centigrade
- Stability over time: Ability of the high voltage supply to hold the high voltage within certain specification over a specified period while keeping other parameters constant. This becomes very critical in the applications like mass spectrometry where the instrument is kept functioning for 8 hours and expected to deliver same results if the same sample is going through. This becomes a problem if the high voltage supply is not stable enough to ensure that.
Designing the circuitry around high voltage in the application
Designing a high voltage circuit board needs extra attention to the details which are not relevant when designing a low voltage or digital circuit board. During the design in process, you may need components around the high voltage supply which becomes challenging especially on the high voltage side. There are certain specifications that become more important when working with high voltage and those are listed below along with the details that need to be taken care of during the high voltage board design.
- Proper creepage and clearance distances from low voltage circuitry
- No ground planes in the high voltage area
- Avoiding sharp edges on the pads on the circuit board
- No silk screens in the vicinity of high voltage
- No plated holes in the high voltage area
- Slots in the board if needed to provide isolation.
- Conformal coating and other insulating materials, if needed
- Key specifications and considerations of the high voltage components
- Voltage and power ratings
- Component derating
- Voltage and temperature coefficients
- Thermal performance
Ease of handling and integration in the end application
Ease of handling and ease of integration are electrical and mechanical aspects of module integration which also need to be considered. Integrating a high voltage becomes easier if the following things are already incorporated in the module that is being designed in.
Protection and Safety
- Input Protections
- Input under and overvoltage protection protects the unit from glitches on the input line.
- Control overvoltage protection protects the unit and the application from getting programmed for higher voltage than it is designed for.
- Output Protections
- Arc protection saves the unit from arcing events in the application.
- Overcurrent protection and short circuit protection provide safety for the module and application in case the unit is overloaded beyond the point it can optimally deliver or it sees a short circuit in the application.
- Thermal Protections
- Thermal Shut- Down protects the module in case it undergoes a temperature range which is beyond its specified operating temperature range. Based on how this is designed, usually it recovers after the over temperature condition has been removed.
Figure 1. Diagram of the device. Image used courtesy of Bodo’s Power Systems
These are the signals used to control and monitor the high voltage module. The digital compatibility of the control and monitor signals is important as that defines what circuitry you will need to control the high voltage. If the signals are DAC compatible, then controlling the high voltage becomes much easier given that the module has that feature available. Measuring high voltage and load current is a challenge because of the special equipment needed as your DMM and scope probes are limited to certain voltage which usually does not go beyond 1000VDC. If the module has the digital compatible monitor signals that makes life much easier for the designer. Some of the signals and their features which comes in handy in module integration are listed below.
- Voltage Programming
- Enable/Disable Input
- Output voltage monitoring
- Load current monitoring.
- Digital compatibility of the control and monitor signals
- Repeatability of the performance
Figure 2. XP Power’s HRC05 DC/DC Converter. Image used courtesy of Bodo’s Power Systems
Physical Module Integration
This corresponds to the space and physical integration of the module in the application circuitry. Following are the things that should be considered.
- Input and output connections
- Size constraints
- Mechanical Layout
- PCB mounting
- Chassis mounting
- Environmental considerations
- Heatsinking capability, if needed
XP Power’s HRC05 is aimed at addressing the accuracy, reliability, and integration issues faced by the system designers and provide an optimal solution in a miniature package.
This article originally appeared in Bodo’s Power Systems magazine.