As microprocessors enhance control capability in power electronics, the ability to manage load current becomes more and more of a doable feature than a complicated nightmare. In this blog, we take a look at an intelligent load management product after starting with the basics of steering as well as sensing output currents.
Steering output currents has evolved with the advances offered by semiconductor switches. For the majority of load management circuits, a MOSFET transistor is quickly replacing the relay as the chosen switch technology. There are two ways to insert a MOSFET transistor into a circuit
- As a high side P-Channel switch
- As a low side N-Channel switch
A quick review for the two MOSFET transistor types reminds us that P-Channel MOSFETs are gated by pulling the gate to a voltage lower than the source whereas N-Channel MOSFETs have the gate activated by a voltage higher than the source. In addition, current flow is opposite. These two factors determine the orientation of the switch in relation to the voltage and current feeding the load.
(Image courtesy of Reference 5)
The following figure shows how a P-Channel is favored as a load switch as it switches in the voltage while the N-Channel switches out the ground [often referred to as “the return”].
In both cases, the gate voltage must exceed the device’s threshold voltage in order to fully turn the device on as a switch in the ohmic region. Note that the discussion from this point focuses on enhancement mode P-Channel and N-Channel MOSFETs. There are various types of JFETs that have different gating requirements.
(Image courtesy of Reference 6)
Transitioning back from device operation to load management circuits, the following image shows a p-FET on the high side [voltage side] as the switching element with an N-Channel efuse product by ON Semi.
(Image courtesy of Reference 4)
The following image shows an n-FET on the low side [return side] as the switching element with an N-Channel efuse product by ON Semi. Although N-channel MOSFETs are roughly one third smaller and therefore less costly than P-Channel versions, P-Channels are preferred for load management due to the ability to keep a ground reference in place.
(Image courtesy of Reference 4)
The efuse concept is an important enhancement as it allows for a circuit to be opened in the event of a reverse polarity, shorted output, or over-current condition. In a similar manner, the current through the switch can also be monitored and controlled. In fact, switch oscillations can occur if gating is not performed correctly as explained in Reference 1.
Although semiconductors don’t exhibit switch bounce like relays do, there is still a potential for unwanted ringing. I’d suggest reading the article titled, High-Side Current Sensing1 , as it has some good pointers showing how to switch smoothly.
From here the blog will focus only on high-side current sensing. Although high-side current sensing can be controlled with analog circuitry1 , digitally controlled high side current control7 is advancing to higher levels. These switches have additional intelligence built in including programmable current levels and digitized current level readings that can be fed back to a microprocessor. This information is stored in a microprocessor in relation to timed sampling of the events thus creating a history of the recorded level. Software is then used to determine changes in load current over time. The information is compared to a programmed threshold with the ability to alert the user of the changes.
In the case of relay loads, this information proves valuable for “heads up” alert to impending component failures. This intelligent load management product7 operates as a single entity or in conjunction with an intelligent power supply. When operating with the intelligent power supply, RS-485 communication is possible for programmable load monitoring and updating on the fly.
Load management enhancements are changing the power industry. As digital control expands, features offering more accurate and adjustable control, system performance and reliability are improved right down to being able to predict failures. This offers advantages in maintenance costs over sending a technician out merely to replace a blown fuse.
- “High-Side Current Sensing”, Aaron Schultz, Analog Devices, RAQ 3/11/2018 on Planet Analog
- “High side driver and Low side driver” Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts.
- And8350.rev0, Alan Ball, ON Semi
- “eFuse Reverse Voltage Protection” prepared by Alan Ball, ON Semi application note AND8350
- PFET high side image reference web address
- NFET low side image reference website
- NU2 Intelligent Low Management Distribution website