Instructions for DIODES INCORPORATED models including: AP74700AQ, AN1193, AN1192, AN1193 Surface Mount Zener Diode Zener Diodes, AN1193, Surface Mount Zener Diode Zener Diodes, Mount Zener Diode Zener Diodes, Zener Diode Zener Diodes, Diode Zener Diodes, Zener Diodes
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DocumentDocumentAN1193
MOSFET Selection Guide for Ideal Diode Controllers
Isaac Sibson and Eduard Santa, Automotive BU, Diodes Incorporated
Wherever there is access to the batteries of a device, there is a risk that users might install or connect them the wrong way around. Replacing batteries is an occasional task complicated by the fact that the two battery terminals can be of similar size and shape. Connecting a car battery incorrectly can be especially damaging. Because of the risk that end-users might connect batteries incorrectly, system designers need to ensure that the extremely expensive electronic modules in a car are not damaged from reverse polarity.
Input reverse polarity protection, also known as input reverse voltage protection, can be achieved using a simple series diode. However, many designers are looking to reduce energy waste in order to achieve more efficient solutions, especially when systems have increasingly higher current demands. At a high current, the power dissipation of the diode could result in it overheating. For more in-depth information on input reverse polarity protection solutions, see application note AN1192: Understanding the Different Approaches to Input Reverse Voltage Protection (RVP).
Today's car infotainment systems have a large bright screen, use a powerful processor, and have multiple channels of audio amplification. With these considerations, power input levels of 300-500W are not at all unrealistic, and all of this is fed from a 12V DC battery. You could be considering currents of 20-40A, maybe even more.
At 20A, a diode with a forward voltage (VF) of 0.7V would dissipate 14W of power, which is undesirable from both thermal management as well as efficiency viewpoints. This is even more important now as we look for ever greater efficiencies, for example, to maximize range in Electric Vehicles (EVs).
A good solution for high-current applications like EVs is the ideal diode controller, such as Diodes Incorporated's (Diodes) AP74700AQ combined with an N-channel MOSFET (nMOS) on the high side (Figure 1). The AP74700AQ will regulate an nMOS as a diode with a VF of 20mV, if possible (explained further in this application note). Otherwise, the MOSFET will be driven to a fully-ON state where the VF is dictated by the drain-source on-resistance (RDS(ON)) of the chosen MOSFET. For example, a 10m MOSFET would have a voltage drop of 0.2V at 20A and dissipate 4W, a significant reduction compared to a diode.
Car Battery Or
DC Rail
(12V, 24V, 48V)
AP74700AQ
Figure 1: Typical Circuit Schematic for the AP74700AQ Ideal Diode Controller
In this application note, we discuss some of the design considerations that should be accounted for in the MOSFET selection process for a high-side ideal diode controller, such as the AP74700AQ.
During normal operation, the current in the MOSFET flows from source to drain. If the MOSFET is not enabled by the ideal diode controller, the current will still be able to flow in the forward direction (i.e. from input to load) through the body diode. Therefore, the AP74700AQ is not able to provide the load switch functionality.
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Voltage Rating All N-channel MOSFET (nMOS) devices used with the AP74700AQ should have a maximum gate-source voltage (VGS) rating of at least ±20V. With that in mind, you can then select the maximum drain-source voltage (VDS) for the MOSFET.
The MOSFET's VDS must be able to withstand the normal battery voltage. However, there could be a case where a large capacitance on the load side could be fully charged (i.e. at +12V). When the input is connected to a reverse polarity (-12V), the voltage is effectively doubled. Therefore, the MOSFET selected should have a VDS of at least twice the maximum nominal voltage rating of the system.
Additionally, standards ISO7637-2 and ISO16750-2 define a series of pulses and spikes that an automotive system must withstand, which means users typically select devices with a higher voltage rating. Consequently for automotive applications, it's recommended to use 40V parts for a 12V nominal rail, 60V parts for a 24V nominal rail, and 80V or 100V parts for a 48V nominal rail.
The AP74700AQ ideal diode controller is rated at 65V (max) and can also withstand a -65V rail voltage. Because of the possibility of a doubled reverse voltage, you may want to choose a 60V MOSFET, even for a 12V nominal rail.
Diodes recommends that the AP74700AQ is combined as follows:
Application Voltage 3.7V
7.4-14.8V 12-14.4V 24-28V
Battery Type Li-Ion
Li-Ion 2-4 cell Lead acid Lead acid
MOSFET VDS 12V 30V 40V 60V
Example Application Handheld device
Cordless power tools 12V automotive
24V commercial vehicle
Table 1: Recommended MOSFET VDS for Different Applications
Current and RDS(ON) The next parameter to decide upon is the RDS(ON) of the MOSFET, which is required to support the load current of the application.
A fully enabled (i.e. VGS = 10V) nMOS will act as a resistor, where the resistance is the RDS(ON) parameter of that device. The voltage drop across the device is calculated by Ohm's law.
The AP74700AQ ideal diode controller will drive the gate of the nMOS to regulate the voltage across its source and drain to a minimum of 20mV at low load currents. At higher currents, the voltage across the nMOS will be limited by the RDS(ON).
Figure 2 illustrates two modes of operation:
1. "Regulated" mode, where the AP74700AQ maintains the 20mV drop across the nMOS 2. "RDS(ON) limited" mode, where the nMOS acts as a resistor
The slope of the RDS(ON) limited mode is determined by the MOSFET chosen.
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Figure 2: Voltage Drop Across Various MOSFETs Against Current
Generally, a lower RDS(ON) of a MOSFET leads to larger die size, larger packaged device, and higher cost. In low-current conditions where the voltage drop across the MOSFET stays regulated around 20mV, there is no advantage of using a very low RDS(ON) device because of the extra cost.
When selecting a MOSFET, begin with the intended load current and then consider any peaks. Must the voltage drop across the MOSFET remain steady, or is it acceptable to have some increase in voltage drop during brief current peaks?
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Figure 3: MOSFET Voltage Ripple with Varying Current
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Figure 3 shows how three different MOSFET devices will behave in conjunction with the AP74700AQ, with a varying current between approximately 1-2A. The three MOSFETs have RDS(ON) values of 10m, 15m, and 25m, and show three different behaviours:
1. The red trace shows a 25m MOSFET that is almost entirely RDS(ON) limited and acts as a resistor, except for the occasional current dip when it hits the 20mV regulation mode.
2. The 15m device (yellow trace) is mostly regulated at 20mV and drops just a bit more voltage when the current peaks above 1.3A.
3. The green trace shows a 10m device, which is almost entirely regulated at 20mV, with only the slightest increase when the current exceeds 2A.
A regular diode would typically drop 700mV. It is clear that the 25m MOSFET represents a significant improvement in efficiency compared to a diode, and while the 10m device improves further, it might not be cost-effective at this current level. This decision on which option to choose is up to the designer: whether the goal is to maximize efficiency or trade-off against cost.
If the load current were 20A, then you would look to lower RDS(ON) devices again, around the 1-3m level.
Table 2 lists suggested MOSFETs that are a good match to selected currents:
VDS (V) 30 40 60
Load Current (A) 3 5 8 10 20 30 8 10 15 20 30 50 5 10 15 20 30
RDS(ON) (m) 25 17 12 7 3.2 1.7 11.5 7.5 5.5 3 1.2 0.7 18 8.3 6 3.1 1.5
Part Number DMN3028LQ DMT3020LFDFQ DMN3016LFDFQ DMT3006LFDFQ DMT3003LFGQ DMTH31M7LPSQ DMTH4008LFDFWQ DMT47M2SFVWQ DMTH45M5LFVWQ DMTH43M8LFGQ DMTH41M2SPSQ DMTH4M70SPGWQ DMTH6016LFDFWQ DMT69M5LFVWQ DMT6007LFGQ DMTH6004LPSQ DMTH61M5SPSWQ
Package SOT23
U-DFN2020-6
PowerDI3333-8 PowerDI5060-8 U-DFN2020-8
PowerDI3333-8
PowerDI5060-8 PowerDI8080-5 U-DFN2020-6 PowerDI3333-8
PowerDI5060-8
Table 2: MOSFET Recommendations for High-Current Applications
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Packaging and Layout The MOSFET package and the amount of copper area on the PCB influence its thermal performance. When selecting a MOSFET to partner with the AP74700AQ, it is worth thinking about the MOSFET package and how it aligns with the pins on the controller.
Figure 4: AP74700AQ Pin Order and Connection to MOSFET Pins The PowerDI®3333 and PowerDI5060 packages easily and neatly connect neatly with the AP74700AQ ( Figure 5, right-side image).
Figure 5: AP74700AQ PCB Layout with SOT23 MOSFET (Left) and PowerDI3333/PowerDI5060 MOSFET (Right)
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However, be aware that some packages are different and may be more difficult to connect. The U-DFN2020-6 package, for example, requires a layer change for the gate (Figure 6, left-side image).
Figure 6: AP74700AQ PCB Layout with DFN-2020 MOSFET (Left) and PowerDI1012 MOSFET (Right) If you are designing a solution for a product range or want to reuse the same circuit blocks again and again in different products, then it is worth selecting a MOSFET footprint that offers a wide range of RDS(ON) and voltage options. The PowerDI3333 is a good choice as it covers a wide range from less than 70m down to 2m, and offers a compact solution. For higher power solutions, the PowerDI5060 can go below 1m, and the PowerDI8080 and PowerDI1012 devices go lower still.
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MOSFET Thermals Once the RDS(ON) has been selected, you need to consider the thermal behavior of the MOSFET. By understanding whether it is operating in the regulated region or if the RDS(ON) is limited, you can determine the worst-case power dissipation as a simple VI (regulated region) or I2R (RDS(ON) limited region) calculation. From that, and the ambient temperature, you can determine the thermal headroom and required junction-to-ambient thermal impedance (RJA), which can be used to determine the most appropriate package.
When using an I2R calculation, it is important to consider the thermal coefficient () of the device. This effect of RDS(ON) increasing with temperature can be seen in Figure 7, which is a graph taken from a MOSFET datasheet.
Figure 7: Example MOSFET ON-state Resistance Variance with Die Temperature
For more information on copper areas and thermal resistances, see Diodes' application note AN1157: Understanding Thermal Resistance in the Real World. The copper area provided and PCB construction (in terms of layers, vias, copper weights, etc.) will have a significant impact on the real-world thermal performance of the system.
Example:
= () - = 175°C 85°C = 90°C
(1)
×
20 × 50
={
× RDS(ON)MAX
×
I2
=
{
1.25 × 0.002 × 20A2
=
1
(2)
=
(1)
(2)
90° 1
=
90 °/
(3)
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Taking some real-life measurements from the DDB104R3 and DDB106R1 demo boards, using some of the 60V MOSFETs detailed in Table 2, yields the following results:
Device TA TD IOUT
P Calc PD
°C/W
DMTH6016LFDFWQ 25°C 42.2°C 5A
1.05 × 0.018 × 52 0.4725W 36.4°C/W
DMT69M5LFVWQ 25°C 65.1°C 10A
1.2 × 0.0083 × 102 0.996W 40.2°C/W
Device TA TD IOUT
P Calc PD
°C/W
DMTH6004LPSQ 25°C
105.6°C 25A
1.4 × 0.0031 × 252 2.7125W 29.7°C/W
DMTH61M5SPSWQ 25°C
129.1°C 35A
1.6 × 0.0015 × 352 2.94W
35.4°C/W
Table 3: Real-world Examples of AP74700AQ Controlling Different MOSFETs
Summary
The ideal diode controller is an efficient solution to providing input reverse polarity protection. Choosing the right MOSFET to go with the ideal diode controller is just as important and, by following some simple steps, it can be quite easy. Diodes Incorporated produces a wide range of appropriate MOSFETs for this purpose, and Table 2 provides a set of suggested devices covering a wide range of load currents.
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IMPORTANT NOTICE
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WITH REGARDS TO ANY INFORMATION CONTAINED IN THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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Diodes Incorporated - Analog and Discrete Power SolutionsDiodes Incorporated - Analog and Discrete Power SolutionsImportant Noticediodes.com/about/company/terms-and-conditions/terms-and-conditions-of-sales/AP74700AQ: +70V, -65V Reverse Voltage Protection Automotive-Compliant Ideal Diode Controller (AP74700AQ)