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Application Note

High voltage scheme of Maxim for industrial ultrasonic equipment

Abstract: at present, most of the ultrasonic emission IC are designed for medical applications and do not necessarily meet the needs of industry. In industrial applications, such as non destructive testing (NDT), flowmeter, sonar, and so on, they usually have different voltage, current and frequency requirements. The high voltage (HV) products of Maxim can be widely used in industrial design because of their great flexibility. Various applications of MAX4940 four road high voltage digital pulse generator and MAX4968 high voltage multiplexer (MUX) are introduced in this paper.


Catalog



Overview


O MAX4940 high voltage digital pulse generator


O MAX4968 high voltage analog switch



Support application


O bipolar pulse


O monopole positive pulse


O monopole negative pulse



Voltage driven BTL architecture


Increase drive current in parallel


High frequency or low frequency applications


O low frequency work (< 1MHz)


O high frequency work (> 20MHz)


Overview

MAX4940 high voltage digital pulse generator


Figure 1 shows the basic function diagram of the MAX4940, which includes the high voltage digital pulse generator of 4 channels (only one of the 4 channels is drawn in the picture).


S1 and S2 switches (respectively connected to VPP_ and VNN_) have 200V voltage and 2A drive capabilities.


The S3 switch (called the clamp switch in data data) has the capability of 200V voltage and 1A current drive.


Digital pulse generator can work in bipolar and unipolar applications, supporting the following applications:


O [VPP, VNN] = [+100V, -100V] bipolar pulse


O [VPP, VNN] = [0, -200V] unipolar negative pulse application


O [VPP, VNN] = [+200V, 0] unipolar positive pulse application


INP_ and INN_ control switches S1 and S2, respectively.


INC_ control switch S3 (clamp), but also restricted by S1 and S2. In most applications, INC_ does not need to be driven, and can always keep it high, but only INP_ and INN_. At this point, every time the S1 and S2 are closed, S3 will be activated.



Figure 1. MAX4940 functional block diagram (one of the 4 channels)



MAX4968 high voltage analog switch


Figure 2 shows a MAX4968 function diagram with a 16 - way independent high voltage analog switch. The internal state of each switch through the SPI interface programming. In most ultrasonic applications, high voltage analog switches are used to implement the high voltage multiplexer.


The swing of SW1A and SW1B can reach VNN to VNN + 200V.


High voltage analog switches can support bipolar and unipolar applications, and the input / output voltage range can be one of the following cases:


O (SW_) range = [+100V, -100V] bipolar


O (SW_) range = [0, -200V] monopole negative pulse


O (SW_) range = [+200V, 0] monopole positive pulse


According to the amplitude and polarity of the input signal, VNN can change from the range of 0V to -200V. VNN can share a negative power supply with a pulse generator (transmission circuit).


VPP uses low voltage power supply (only 10V).


The equivalent RON is flat in the entire input range (about 20 omega) and the conduction capacitance is 16pF.



Figure 2. MAX4968 functional block diagram



Support application

Maxim‘s high voltage pulse generator and switch are designed with unique design and can work in bipolar and unipolar applications (most industrial ultrasonic applications are used as monopole). The Maxim solution can reduce the overall size and simplify the system, especially in unipolar applications. The functions and sequence diagrams of bipolar and unipolar emission are shown below. External interference suppression diodes can be omitted in some cases.



Bipolar pulse



Figure 3. MAX4940 typical 4 channel bipolar digital pulse generator



Figure 4. uses MAX4940 and MAX4968 bipolar applications to greatly simplify the design of high voltage power supply


Note:


1. for convenience, only two channels are connected to MAX4968 and are configured as 1:2 high voltage MUX.


The 2. only needs two high voltage power sources (VPP, VNN).


The 3. MAX4940 bare welded plate (Figure 4 is not drawn) must be connected to VNN.



Figure 5. pulse and switch signal waveforms in bipolar and negative pulse applications of MAX4940 and MAX4968


Note: CLP_ is always high. Each channel needs only 2 control signals, providing 3 level launches.



Monopole positive pulse



The application of figure 6. MAX4940 in monopole positive pulse



Figure 7. applications of MAX4940 and MAX4968 in monopole to reduce the demand for high voltage power supply


Note:


1. only need a high voltage power supply.


2. note that the CGN_ can be directly connected to the CDN_, allowing each channel to save a capacitor.


3. MAX4940 no longer needs VEE (VEE grounding).


The voltage of 4. signal coupling capacitance is more than 200V.


5. MAX4940 bare welded disks (not shown) need to be grounded.



Fig. 8. MAX4940 and MAX4968 signal sequence diagram in unipolar positive pulse application



Monopole negative pulse



The unipolar application of figure 9. MAX4940



Figure 10. applications of MAX4968 and MAX4940 in monopole negative pulses to reduce the demand for high voltage power supply


Note:


1. only need a high pressure.


2. MAX4940 naked pads (unexpressed) need to connect to VNN.


3., the unipolar positive pulse (FIGS. 6 and 7) has a slight advantage over unipolar negative pulse architecture (less external components and bare pads connected to GND can improve heat dissipation).



Figure 11. MAX4940 and MAX4968 signal waveforms in unipolar negative pulse applications



BTL architecture driven by voltage doubling

In industrial applications, it is often required to drive more than 200V transmitters. Nondestructive testing, flowmeter or other applications, the transmitter used may require more than 200V pulse drive.


The MAX4940 can drive a transmitter through a BTL to produce a peak of two times the peak of the excitation signal. Two MAX4940 channels are required to drive a transmitter. The excitation voltage can be as high as 400VP-P.


The BTL configuration needs to use the two electrodes of the transmitter, which is not applicable to the large transmitter array connected to the GND in the common end of each unit.


Figure 12 shows a typical BTL application block diagram, using a unipolar configuration (in this case it is a positive pulse). Transmission

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